DEPARTMENT OF CHEMISTRY UNIVERSITY OF SWAZILAND MAY 2017 FINAL EXAMINATION TITLE OF PAPER COURSE NUMBER TIME Important Information INTRODUCTION TO ANALYTICAL CHEMISTRY C204/ CHE 212 3 HOURS 1. Each question is worth 25 marks. 2. Answer any four (4) questions in this paper. 3. Marks for A.LL procedural calculations will be awarded. 4. Start each question on a fresh page of the answer sheet. 5. Diagrams must be large and clearly labelled accordingly. 6. This paper contains an appendix of chemical constants 7. Additional material: graph paper. You are not supposed to open this paper until permission has been granted by the chief Invigilator.
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DEPARTMENT OF CHEMISTRY
UNIVERSITY OF SWAZILAND
MAY 2017 FINAL EXAMINATION
TITLE OF PAPER
COURSE NUMBER
TIME
Important Information
INTRODUCTION TO ANALYTICAL CHEMISTRY
C204 CHE 212
3 HOURS
1 Each question is worth 25 marks
2 Answer any four (4) questions in this paper
3 Marks for ALL procedural calculations will be awarded
4 Start each question on a fresh page ofthe answer sheet
5 Diagrams must be large and clearly labelled accordingly
6 This paper contains an appendix of chemical constants
7 Additional material graph paper
You are not supposed to open this paper until permission has been granted by the chief Invigilator
QUESTION 1 [25 Marks]
a) The molarity of a NaOH solution was determined by titration vs KHP Individual titrations gave the following concentration 01127M 01 126M 01 132M 01l74M and 01 173M
1 Can any of the points be rejected at the 90 confidence level If so which one(s) [4]
n For all remaining points calculate the mean the standard deviation the relative standard deviation in ppt and the confidence interval at the 90 confidence level [6]
lll This particular analysis has been performed thousands of times - enough such that the population standard deviation can be determined Assuming that the population standard deviation is equal to 00003M for this determination calculate the new confidence interval at the 90 confidence level [3]
b) A particular water soluble fertiliser contains phosphorus in the form of phosphate ions polo A student used the following procedure to determine the
percentage of phosphorus in a sample of soluble fertiliser
- 517 g of fertiliser was added to a 2500 mL volumetric flask and water was added to make it up to the mark
- 2000 mL of this solution was pipetted into a conical flask A slight excess of precipitating agent was added to precipitate the phosphate ions as MgNH4P04bull
- The precipitate was filtered washed with water and then converted by heating into Mg2P20 7bull The mass of Mg2P20 7 was 00312 g
Calculate the percentage by mass ofphosphate in the fertilizer [7]
c) What is meant by digestion of a precipitate Briefly describe what happens in the process of digesting a precipitate and give two (2) advantages of this step during gravimetric analysis [3]
d) What is peptization How can this phenomenon be avoided during gravimetric analysis [2]
1
QUESTION 2 [25 Marks]
a) Consider the titration ofa 200mL sample of0105M HCN with Ol25M NaOH Determine the following Ka HCN 49 x 10-10
1 Initial pH [2] 11 The volume of added base required to reach equivalence point in mL
[1] 111 The pH at 100 mL of added base [4] IV The pH at equivalence point [3] v The pH after adding 50 mL of base beyond the equivalence point [2]
b) Sketch the titration curve for the titration in (a) and clearly marking the initial pH buffer region equivalence point in your diagram [3]
c) With the aid of an accompanying diagram describe the prcedure for the analysis of a sample using multiple point standard addition method [8]
d) What is the main advantage of the method in 2( c) over external standard calibration method [2]
QUESTION 3 [25 Marksl
a) In the process of assessing responsibility for an oil spill at the Lusushwana River two possible suspects are identified To differentiate between the two samples of oil the ratio of the concentration for two polyaromatic hydrocarbons is measured using fluorescence spectroscopy These values are then compared to the sample obtained from Lusushwana River
1 Can either (or both) suspects be eliminated based on the results of the analysis at 99 confidence level Clearly show working and how you reached your conclusion [8]
11 From the above results should there be a concern that any combination of the standard deviation values demonstrates a significant difference [3]
2
b) Using examples differentiate between quantitative and qualitative analysis in analytical chemistry [4]
c) During your laboratory sessions for most of the experiments you first had to standardize the titrant
i) What is standardization and why is it necessary that a titrant be standardized before use [4]
ii) Give an example of an acid you used in the lab to standardize a base and one example of a base you used to standardize an acid [2]
d) In the determination of chlorine by Fajans titration in samples
i) Name the common adsorption indicator used in this titration [1] ii) What is the reason for the addition of dextrin before titration [1]
e) Which data set is the more precise A or B Explain (2)
B
3
QUESTION 4 [25 Marks)
a) The CO in a 203 L sample of gas was converted to CO2 by passing the gas over iodine pentoxide heated to 150 degrees Celsius
hOs (s) + 5CO (g) -+ 5C02(g) + h (g)
The iodine was distilled at this temperature and was collected III an absorber containing 825 mL of 001101 M Na2S203
12 (g) + 2s2ol- (aq) -+ 21 (aq) + s4ol- (aq)
The excess Na2S203 was back titrated with 216 mL of 000947 M 12 solution
i) Calculate the concentration in milligrams of CO (2801 glmol) per litre of sample [5]
ii) The method used in a(i) is known as back titration explain what is meant by back titration [2]
iii) Give four (4) purposes of back titration ie scenarios which would require the use ofback titration instead of direct titration [4]
b) In titrimetry i) Differentiate between primary standard and a secondary standard for
titrimetric analysis [2] ii) Give four (4) desirable properties for a primary standard used for
titration purposes [4]
c) i) List the four main types of determinate error [2]
ii) Give a brief explanationdescription of each of the types of determinate error you listed in c (i) giving a specific example for each [4]
iii) Explain two ways which can be used to detect determinate errors [2]
4
QUESTION 5 [25 Marks]
a)
i) Explain the term Homogeneous precipitation in gravimetry [1]
ii) Explain two ways in which homogeneous precipitation can be achieved
during gravimetric analysis Give a specific example for each [4]
iii) What are the unique advantages of homogenous precipitation when
compared to direct precipitation [3]
iv) What is meant by co-precipitation in gravimetry [2]
v) Briefly describe three different types of co-precipitation [3]
b) A mass of 2473 g of an unknown sample was dissolved in 10 mL concentrated
H2S04 and the mixture boiled for 5 minutes The solution was cooled and the volume
made up to 250 mL in a volumetric flask A series of potassium standards gave the
following intensities
Standard (JlgmL) Emission Intensity
Blank o 500 124
1000 243
2000 486
3000
1 If the emission intensity of the sample was found to be 417 find the
concentration of potassium in the unknown in Ilglg using the least squares
method [10]
11 If the potassium solution used to calibrate the instrument in b(i) was only
98 pure (in terms of analyte) what effect would have on the accuracy and
precision on the analyses of the sample [2]
5
QUESTION 6 [25 Marks)
a) An analysis is carried out in water over five days to determine the concentration of Cu a river passing through the Matsapha industrial site After every 10 samples the analyst ran a control sample and the mean concentration for each day of the control is shown in the table below i) Plot the quality control chart [3] ii) Using this information advice the analyst on the quality of data
obtained [4]
Concentration Day (ppm)
I 1 329
2 333
I 3 345 4 326 5 322
Polpulation
Mean 331
Std I Deviation 9
b) A chemistry student needs 250 mL of a solution buffered at a pH of 1100 How many grams of ammonium chloride have to be added to 250 mL of 02 mollL NH3 to make such a buffer (Volume is assumed not to change) Kb 18 x 10-5
bull [5]
c) This question is about a buffer solution made by mixing together solutions containing ethanoic acid and sodium ethanoate
i) If you add a small amount of an acid such as dilute hydrochloric acid to this solution the pH doesnt change much Explain what happens to the extra hydrogen ions you have added [21
ii) If you add a small amount of an alkali such as sodium hydroxide solution to the buffer solution again the pH doesnt change much Explain what happens to the extra hydroxide ions you have added [2]
d) The concept of CRM and or SRM is widely used by industry for their quality control measures Briefly explain
i) What are CRM or SRMs [I]
6
ii) What is their central role in analytical chemistry [2]
iii) How are they certified [2]
e) Distinguish between the following
i) Systematic and random errors [2]
ii) Precision and accuracy [2]
7
APPENDIX
Useful Formulas
8
TABLES
TABLE 1 Table of Acid and Base Strength
I Ka Acid Coniu~ate Base Name Formula Formula Name
Large Perchloric acid HCI04 CI04 Perchlorate ion
i 32 10 Hydroiodic acid HI 1shy Iodide 10 10) Hydrobromic acid HBr Br- Bromide I 13 10
a) The molarity of a NaOH solution was determined by titration vs KHP Individual titrations gave the following concentration 01127M 01 126M 01 132M 01l74M and 01 173M
1 Can any of the points be rejected at the 90 confidence level If so which one(s) [4]
n For all remaining points calculate the mean the standard deviation the relative standard deviation in ppt and the confidence interval at the 90 confidence level [6]
lll This particular analysis has been performed thousands of times - enough such that the population standard deviation can be determined Assuming that the population standard deviation is equal to 00003M for this determination calculate the new confidence interval at the 90 confidence level [3]
b) A particular water soluble fertiliser contains phosphorus in the form of phosphate ions polo A student used the following procedure to determine the
percentage of phosphorus in a sample of soluble fertiliser
- 517 g of fertiliser was added to a 2500 mL volumetric flask and water was added to make it up to the mark
- 2000 mL of this solution was pipetted into a conical flask A slight excess of precipitating agent was added to precipitate the phosphate ions as MgNH4P04bull
- The precipitate was filtered washed with water and then converted by heating into Mg2P20 7bull The mass of Mg2P20 7 was 00312 g
Calculate the percentage by mass ofphosphate in the fertilizer [7]
c) What is meant by digestion of a precipitate Briefly describe what happens in the process of digesting a precipitate and give two (2) advantages of this step during gravimetric analysis [3]
d) What is peptization How can this phenomenon be avoided during gravimetric analysis [2]
1
QUESTION 2 [25 Marks]
a) Consider the titration ofa 200mL sample of0105M HCN with Ol25M NaOH Determine the following Ka HCN 49 x 10-10
1 Initial pH [2] 11 The volume of added base required to reach equivalence point in mL
[1] 111 The pH at 100 mL of added base [4] IV The pH at equivalence point [3] v The pH after adding 50 mL of base beyond the equivalence point [2]
b) Sketch the titration curve for the titration in (a) and clearly marking the initial pH buffer region equivalence point in your diagram [3]
c) With the aid of an accompanying diagram describe the prcedure for the analysis of a sample using multiple point standard addition method [8]
d) What is the main advantage of the method in 2( c) over external standard calibration method [2]
QUESTION 3 [25 Marksl
a) In the process of assessing responsibility for an oil spill at the Lusushwana River two possible suspects are identified To differentiate between the two samples of oil the ratio of the concentration for two polyaromatic hydrocarbons is measured using fluorescence spectroscopy These values are then compared to the sample obtained from Lusushwana River
1 Can either (or both) suspects be eliminated based on the results of the analysis at 99 confidence level Clearly show working and how you reached your conclusion [8]
11 From the above results should there be a concern that any combination of the standard deviation values demonstrates a significant difference [3]
2
b) Using examples differentiate between quantitative and qualitative analysis in analytical chemistry [4]
c) During your laboratory sessions for most of the experiments you first had to standardize the titrant
i) What is standardization and why is it necessary that a titrant be standardized before use [4]
ii) Give an example of an acid you used in the lab to standardize a base and one example of a base you used to standardize an acid [2]
d) In the determination of chlorine by Fajans titration in samples
i) Name the common adsorption indicator used in this titration [1] ii) What is the reason for the addition of dextrin before titration [1]
e) Which data set is the more precise A or B Explain (2)
B
3
QUESTION 4 [25 Marks)
a) The CO in a 203 L sample of gas was converted to CO2 by passing the gas over iodine pentoxide heated to 150 degrees Celsius
hOs (s) + 5CO (g) -+ 5C02(g) + h (g)
The iodine was distilled at this temperature and was collected III an absorber containing 825 mL of 001101 M Na2S203
12 (g) + 2s2ol- (aq) -+ 21 (aq) + s4ol- (aq)
The excess Na2S203 was back titrated with 216 mL of 000947 M 12 solution
i) Calculate the concentration in milligrams of CO (2801 glmol) per litre of sample [5]
ii) The method used in a(i) is known as back titration explain what is meant by back titration [2]
iii) Give four (4) purposes of back titration ie scenarios which would require the use ofback titration instead of direct titration [4]
b) In titrimetry i) Differentiate between primary standard and a secondary standard for
titrimetric analysis [2] ii) Give four (4) desirable properties for a primary standard used for
titration purposes [4]
c) i) List the four main types of determinate error [2]
ii) Give a brief explanationdescription of each of the types of determinate error you listed in c (i) giving a specific example for each [4]
iii) Explain two ways which can be used to detect determinate errors [2]
4
QUESTION 5 [25 Marks]
a)
i) Explain the term Homogeneous precipitation in gravimetry [1]
ii) Explain two ways in which homogeneous precipitation can be achieved
during gravimetric analysis Give a specific example for each [4]
iii) What are the unique advantages of homogenous precipitation when
compared to direct precipitation [3]
iv) What is meant by co-precipitation in gravimetry [2]
v) Briefly describe three different types of co-precipitation [3]
b) A mass of 2473 g of an unknown sample was dissolved in 10 mL concentrated
H2S04 and the mixture boiled for 5 minutes The solution was cooled and the volume
made up to 250 mL in a volumetric flask A series of potassium standards gave the
following intensities
Standard (JlgmL) Emission Intensity
Blank o 500 124
1000 243
2000 486
3000
1 If the emission intensity of the sample was found to be 417 find the
concentration of potassium in the unknown in Ilglg using the least squares
method [10]
11 If the potassium solution used to calibrate the instrument in b(i) was only
98 pure (in terms of analyte) what effect would have on the accuracy and
precision on the analyses of the sample [2]
5
QUESTION 6 [25 Marks)
a) An analysis is carried out in water over five days to determine the concentration of Cu a river passing through the Matsapha industrial site After every 10 samples the analyst ran a control sample and the mean concentration for each day of the control is shown in the table below i) Plot the quality control chart [3] ii) Using this information advice the analyst on the quality of data
obtained [4]
Concentration Day (ppm)
I 1 329
2 333
I 3 345 4 326 5 322
Polpulation
Mean 331
Std I Deviation 9
b) A chemistry student needs 250 mL of a solution buffered at a pH of 1100 How many grams of ammonium chloride have to be added to 250 mL of 02 mollL NH3 to make such a buffer (Volume is assumed not to change) Kb 18 x 10-5
bull [5]
c) This question is about a buffer solution made by mixing together solutions containing ethanoic acid and sodium ethanoate
i) If you add a small amount of an acid such as dilute hydrochloric acid to this solution the pH doesnt change much Explain what happens to the extra hydrogen ions you have added [21
ii) If you add a small amount of an alkali such as sodium hydroxide solution to the buffer solution again the pH doesnt change much Explain what happens to the extra hydroxide ions you have added [2]
d) The concept of CRM and or SRM is widely used by industry for their quality control measures Briefly explain
i) What are CRM or SRMs [I]
6
ii) What is their central role in analytical chemistry [2]
iii) How are they certified [2]
e) Distinguish between the following
i) Systematic and random errors [2]
ii) Precision and accuracy [2]
7
APPENDIX
Useful Formulas
8
TABLES
TABLE 1 Table of Acid and Base Strength
I Ka Acid Coniu~ate Base Name Formula Formula Name
Large Perchloric acid HCI04 CI04 Perchlorate ion
i 32 10 Hydroiodic acid HI 1shy Iodide 10 10) Hydrobromic acid HBr Br- Bromide I 13 10
a) Consider the titration ofa 200mL sample of0105M HCN with Ol25M NaOH Determine the following Ka HCN 49 x 10-10
1 Initial pH [2] 11 The volume of added base required to reach equivalence point in mL
[1] 111 The pH at 100 mL of added base [4] IV The pH at equivalence point [3] v The pH after adding 50 mL of base beyond the equivalence point [2]
b) Sketch the titration curve for the titration in (a) and clearly marking the initial pH buffer region equivalence point in your diagram [3]
c) With the aid of an accompanying diagram describe the prcedure for the analysis of a sample using multiple point standard addition method [8]
d) What is the main advantage of the method in 2( c) over external standard calibration method [2]
QUESTION 3 [25 Marksl
a) In the process of assessing responsibility for an oil spill at the Lusushwana River two possible suspects are identified To differentiate between the two samples of oil the ratio of the concentration for two polyaromatic hydrocarbons is measured using fluorescence spectroscopy These values are then compared to the sample obtained from Lusushwana River
1 Can either (or both) suspects be eliminated based on the results of the analysis at 99 confidence level Clearly show working and how you reached your conclusion [8]
11 From the above results should there be a concern that any combination of the standard deviation values demonstrates a significant difference [3]
2
b) Using examples differentiate between quantitative and qualitative analysis in analytical chemistry [4]
c) During your laboratory sessions for most of the experiments you first had to standardize the titrant
i) What is standardization and why is it necessary that a titrant be standardized before use [4]
ii) Give an example of an acid you used in the lab to standardize a base and one example of a base you used to standardize an acid [2]
d) In the determination of chlorine by Fajans titration in samples
i) Name the common adsorption indicator used in this titration [1] ii) What is the reason for the addition of dextrin before titration [1]
e) Which data set is the more precise A or B Explain (2)
B
3
QUESTION 4 [25 Marks)
a) The CO in a 203 L sample of gas was converted to CO2 by passing the gas over iodine pentoxide heated to 150 degrees Celsius
hOs (s) + 5CO (g) -+ 5C02(g) + h (g)
The iodine was distilled at this temperature and was collected III an absorber containing 825 mL of 001101 M Na2S203
12 (g) + 2s2ol- (aq) -+ 21 (aq) + s4ol- (aq)
The excess Na2S203 was back titrated with 216 mL of 000947 M 12 solution
i) Calculate the concentration in milligrams of CO (2801 glmol) per litre of sample [5]
ii) The method used in a(i) is known as back titration explain what is meant by back titration [2]
iii) Give four (4) purposes of back titration ie scenarios which would require the use ofback titration instead of direct titration [4]
b) In titrimetry i) Differentiate between primary standard and a secondary standard for
titrimetric analysis [2] ii) Give four (4) desirable properties for a primary standard used for
titration purposes [4]
c) i) List the four main types of determinate error [2]
ii) Give a brief explanationdescription of each of the types of determinate error you listed in c (i) giving a specific example for each [4]
iii) Explain two ways which can be used to detect determinate errors [2]
4
QUESTION 5 [25 Marks]
a)
i) Explain the term Homogeneous precipitation in gravimetry [1]
ii) Explain two ways in which homogeneous precipitation can be achieved
during gravimetric analysis Give a specific example for each [4]
iii) What are the unique advantages of homogenous precipitation when
compared to direct precipitation [3]
iv) What is meant by co-precipitation in gravimetry [2]
v) Briefly describe three different types of co-precipitation [3]
b) A mass of 2473 g of an unknown sample was dissolved in 10 mL concentrated
H2S04 and the mixture boiled for 5 minutes The solution was cooled and the volume
made up to 250 mL in a volumetric flask A series of potassium standards gave the
following intensities
Standard (JlgmL) Emission Intensity
Blank o 500 124
1000 243
2000 486
3000
1 If the emission intensity of the sample was found to be 417 find the
concentration of potassium in the unknown in Ilglg using the least squares
method [10]
11 If the potassium solution used to calibrate the instrument in b(i) was only
98 pure (in terms of analyte) what effect would have on the accuracy and
precision on the analyses of the sample [2]
5
QUESTION 6 [25 Marks)
a) An analysis is carried out in water over five days to determine the concentration of Cu a river passing through the Matsapha industrial site After every 10 samples the analyst ran a control sample and the mean concentration for each day of the control is shown in the table below i) Plot the quality control chart [3] ii) Using this information advice the analyst on the quality of data
obtained [4]
Concentration Day (ppm)
I 1 329
2 333
I 3 345 4 326 5 322
Polpulation
Mean 331
Std I Deviation 9
b) A chemistry student needs 250 mL of a solution buffered at a pH of 1100 How many grams of ammonium chloride have to be added to 250 mL of 02 mollL NH3 to make such a buffer (Volume is assumed not to change) Kb 18 x 10-5
bull [5]
c) This question is about a buffer solution made by mixing together solutions containing ethanoic acid and sodium ethanoate
i) If you add a small amount of an acid such as dilute hydrochloric acid to this solution the pH doesnt change much Explain what happens to the extra hydrogen ions you have added [21
ii) If you add a small amount of an alkali such as sodium hydroxide solution to the buffer solution again the pH doesnt change much Explain what happens to the extra hydroxide ions you have added [2]
d) The concept of CRM and or SRM is widely used by industry for their quality control measures Briefly explain
i) What are CRM or SRMs [I]
6
ii) What is their central role in analytical chemistry [2]
iii) How are they certified [2]
e) Distinguish between the following
i) Systematic and random errors [2]
ii) Precision and accuracy [2]
7
APPENDIX
Useful Formulas
8
TABLES
TABLE 1 Table of Acid and Base Strength
I Ka Acid Coniu~ate Base Name Formula Formula Name
Large Perchloric acid HCI04 CI04 Perchlorate ion
i 32 10 Hydroiodic acid HI 1shy Iodide 10 10) Hydrobromic acid HBr Br- Bromide I 13 10
b) Using examples differentiate between quantitative and qualitative analysis in analytical chemistry [4]
c) During your laboratory sessions for most of the experiments you first had to standardize the titrant
i) What is standardization and why is it necessary that a titrant be standardized before use [4]
ii) Give an example of an acid you used in the lab to standardize a base and one example of a base you used to standardize an acid [2]
d) In the determination of chlorine by Fajans titration in samples
i) Name the common adsorption indicator used in this titration [1] ii) What is the reason for the addition of dextrin before titration [1]
e) Which data set is the more precise A or B Explain (2)
B
3
QUESTION 4 [25 Marks)
a) The CO in a 203 L sample of gas was converted to CO2 by passing the gas over iodine pentoxide heated to 150 degrees Celsius
hOs (s) + 5CO (g) -+ 5C02(g) + h (g)
The iodine was distilled at this temperature and was collected III an absorber containing 825 mL of 001101 M Na2S203
12 (g) + 2s2ol- (aq) -+ 21 (aq) + s4ol- (aq)
The excess Na2S203 was back titrated with 216 mL of 000947 M 12 solution
i) Calculate the concentration in milligrams of CO (2801 glmol) per litre of sample [5]
ii) The method used in a(i) is known as back titration explain what is meant by back titration [2]
iii) Give four (4) purposes of back titration ie scenarios which would require the use ofback titration instead of direct titration [4]
b) In titrimetry i) Differentiate between primary standard and a secondary standard for
titrimetric analysis [2] ii) Give four (4) desirable properties for a primary standard used for
titration purposes [4]
c) i) List the four main types of determinate error [2]
ii) Give a brief explanationdescription of each of the types of determinate error you listed in c (i) giving a specific example for each [4]
iii) Explain two ways which can be used to detect determinate errors [2]
4
QUESTION 5 [25 Marks]
a)
i) Explain the term Homogeneous precipitation in gravimetry [1]
ii) Explain two ways in which homogeneous precipitation can be achieved
during gravimetric analysis Give a specific example for each [4]
iii) What are the unique advantages of homogenous precipitation when
compared to direct precipitation [3]
iv) What is meant by co-precipitation in gravimetry [2]
v) Briefly describe three different types of co-precipitation [3]
b) A mass of 2473 g of an unknown sample was dissolved in 10 mL concentrated
H2S04 and the mixture boiled for 5 minutes The solution was cooled and the volume
made up to 250 mL in a volumetric flask A series of potassium standards gave the
following intensities
Standard (JlgmL) Emission Intensity
Blank o 500 124
1000 243
2000 486
3000
1 If the emission intensity of the sample was found to be 417 find the
concentration of potassium in the unknown in Ilglg using the least squares
method [10]
11 If the potassium solution used to calibrate the instrument in b(i) was only
98 pure (in terms of analyte) what effect would have on the accuracy and
precision on the analyses of the sample [2]
5
QUESTION 6 [25 Marks)
a) An analysis is carried out in water over five days to determine the concentration of Cu a river passing through the Matsapha industrial site After every 10 samples the analyst ran a control sample and the mean concentration for each day of the control is shown in the table below i) Plot the quality control chart [3] ii) Using this information advice the analyst on the quality of data
obtained [4]
Concentration Day (ppm)
I 1 329
2 333
I 3 345 4 326 5 322
Polpulation
Mean 331
Std I Deviation 9
b) A chemistry student needs 250 mL of a solution buffered at a pH of 1100 How many grams of ammonium chloride have to be added to 250 mL of 02 mollL NH3 to make such a buffer (Volume is assumed not to change) Kb 18 x 10-5
bull [5]
c) This question is about a buffer solution made by mixing together solutions containing ethanoic acid and sodium ethanoate
i) If you add a small amount of an acid such as dilute hydrochloric acid to this solution the pH doesnt change much Explain what happens to the extra hydrogen ions you have added [21
ii) If you add a small amount of an alkali such as sodium hydroxide solution to the buffer solution again the pH doesnt change much Explain what happens to the extra hydroxide ions you have added [2]
d) The concept of CRM and or SRM is widely used by industry for their quality control measures Briefly explain
i) What are CRM or SRMs [I]
6
ii) What is their central role in analytical chemistry [2]
iii) How are they certified [2]
e) Distinguish between the following
i) Systematic and random errors [2]
ii) Precision and accuracy [2]
7
APPENDIX
Useful Formulas
8
TABLES
TABLE 1 Table of Acid and Base Strength
I Ka Acid Coniu~ate Base Name Formula Formula Name
Large Perchloric acid HCI04 CI04 Perchlorate ion
i 32 10 Hydroiodic acid HI 1shy Iodide 10 10) Hydrobromic acid HBr Br- Bromide I 13 10
a) The CO in a 203 L sample of gas was converted to CO2 by passing the gas over iodine pentoxide heated to 150 degrees Celsius
hOs (s) + 5CO (g) -+ 5C02(g) + h (g)
The iodine was distilled at this temperature and was collected III an absorber containing 825 mL of 001101 M Na2S203
12 (g) + 2s2ol- (aq) -+ 21 (aq) + s4ol- (aq)
The excess Na2S203 was back titrated with 216 mL of 000947 M 12 solution
i) Calculate the concentration in milligrams of CO (2801 glmol) per litre of sample [5]
ii) The method used in a(i) is known as back titration explain what is meant by back titration [2]
iii) Give four (4) purposes of back titration ie scenarios which would require the use ofback titration instead of direct titration [4]
b) In titrimetry i) Differentiate between primary standard and a secondary standard for
titrimetric analysis [2] ii) Give four (4) desirable properties for a primary standard used for
titration purposes [4]
c) i) List the four main types of determinate error [2]
ii) Give a brief explanationdescription of each of the types of determinate error you listed in c (i) giving a specific example for each [4]
iii) Explain two ways which can be used to detect determinate errors [2]
4
QUESTION 5 [25 Marks]
a)
i) Explain the term Homogeneous precipitation in gravimetry [1]
ii) Explain two ways in which homogeneous precipitation can be achieved
during gravimetric analysis Give a specific example for each [4]
iii) What are the unique advantages of homogenous precipitation when
compared to direct precipitation [3]
iv) What is meant by co-precipitation in gravimetry [2]
v) Briefly describe three different types of co-precipitation [3]
b) A mass of 2473 g of an unknown sample was dissolved in 10 mL concentrated
H2S04 and the mixture boiled for 5 minutes The solution was cooled and the volume
made up to 250 mL in a volumetric flask A series of potassium standards gave the
following intensities
Standard (JlgmL) Emission Intensity
Blank o 500 124
1000 243
2000 486
3000
1 If the emission intensity of the sample was found to be 417 find the
concentration of potassium in the unknown in Ilglg using the least squares
method [10]
11 If the potassium solution used to calibrate the instrument in b(i) was only
98 pure (in terms of analyte) what effect would have on the accuracy and
precision on the analyses of the sample [2]
5
QUESTION 6 [25 Marks)
a) An analysis is carried out in water over five days to determine the concentration of Cu a river passing through the Matsapha industrial site After every 10 samples the analyst ran a control sample and the mean concentration for each day of the control is shown in the table below i) Plot the quality control chart [3] ii) Using this information advice the analyst on the quality of data
obtained [4]
Concentration Day (ppm)
I 1 329
2 333
I 3 345 4 326 5 322
Polpulation
Mean 331
Std I Deviation 9
b) A chemistry student needs 250 mL of a solution buffered at a pH of 1100 How many grams of ammonium chloride have to be added to 250 mL of 02 mollL NH3 to make such a buffer (Volume is assumed not to change) Kb 18 x 10-5
bull [5]
c) This question is about a buffer solution made by mixing together solutions containing ethanoic acid and sodium ethanoate
i) If you add a small amount of an acid such as dilute hydrochloric acid to this solution the pH doesnt change much Explain what happens to the extra hydrogen ions you have added [21
ii) If you add a small amount of an alkali such as sodium hydroxide solution to the buffer solution again the pH doesnt change much Explain what happens to the extra hydroxide ions you have added [2]
d) The concept of CRM and or SRM is widely used by industry for their quality control measures Briefly explain
i) What are CRM or SRMs [I]
6
ii) What is their central role in analytical chemistry [2]
iii) How are they certified [2]
e) Distinguish between the following
i) Systematic and random errors [2]
ii) Precision and accuracy [2]
7
APPENDIX
Useful Formulas
8
TABLES
TABLE 1 Table of Acid and Base Strength
I Ka Acid Coniu~ate Base Name Formula Formula Name
Large Perchloric acid HCI04 CI04 Perchlorate ion
i 32 10 Hydroiodic acid HI 1shy Iodide 10 10) Hydrobromic acid HBr Br- Bromide I 13 10
i) Explain the term Homogeneous precipitation in gravimetry [1]
ii) Explain two ways in which homogeneous precipitation can be achieved
during gravimetric analysis Give a specific example for each [4]
iii) What are the unique advantages of homogenous precipitation when
compared to direct precipitation [3]
iv) What is meant by co-precipitation in gravimetry [2]
v) Briefly describe three different types of co-precipitation [3]
b) A mass of 2473 g of an unknown sample was dissolved in 10 mL concentrated
H2S04 and the mixture boiled for 5 minutes The solution was cooled and the volume
made up to 250 mL in a volumetric flask A series of potassium standards gave the
following intensities
Standard (JlgmL) Emission Intensity
Blank o 500 124
1000 243
2000 486
3000
1 If the emission intensity of the sample was found to be 417 find the
concentration of potassium in the unknown in Ilglg using the least squares
method [10]
11 If the potassium solution used to calibrate the instrument in b(i) was only
98 pure (in terms of analyte) what effect would have on the accuracy and
precision on the analyses of the sample [2]
5
QUESTION 6 [25 Marks)
a) An analysis is carried out in water over five days to determine the concentration of Cu a river passing through the Matsapha industrial site After every 10 samples the analyst ran a control sample and the mean concentration for each day of the control is shown in the table below i) Plot the quality control chart [3] ii) Using this information advice the analyst on the quality of data
obtained [4]
Concentration Day (ppm)
I 1 329
2 333
I 3 345 4 326 5 322
Polpulation
Mean 331
Std I Deviation 9
b) A chemistry student needs 250 mL of a solution buffered at a pH of 1100 How many grams of ammonium chloride have to be added to 250 mL of 02 mollL NH3 to make such a buffer (Volume is assumed not to change) Kb 18 x 10-5
bull [5]
c) This question is about a buffer solution made by mixing together solutions containing ethanoic acid and sodium ethanoate
i) If you add a small amount of an acid such as dilute hydrochloric acid to this solution the pH doesnt change much Explain what happens to the extra hydrogen ions you have added [21
ii) If you add a small amount of an alkali such as sodium hydroxide solution to the buffer solution again the pH doesnt change much Explain what happens to the extra hydroxide ions you have added [2]
d) The concept of CRM and or SRM is widely used by industry for their quality control measures Briefly explain
i) What are CRM or SRMs [I]
6
ii) What is their central role in analytical chemistry [2]
iii) How are they certified [2]
e) Distinguish between the following
i) Systematic and random errors [2]
ii) Precision and accuracy [2]
7
APPENDIX
Useful Formulas
8
TABLES
TABLE 1 Table of Acid and Base Strength
I Ka Acid Coniu~ate Base Name Formula Formula Name
Large Perchloric acid HCI04 CI04 Perchlorate ion
i 32 10 Hydroiodic acid HI 1shy Iodide 10 10) Hydrobromic acid HBr Br- Bromide I 13 10
a) An analysis is carried out in water over five days to determine the concentration of Cu a river passing through the Matsapha industrial site After every 10 samples the analyst ran a control sample and the mean concentration for each day of the control is shown in the table below i) Plot the quality control chart [3] ii) Using this information advice the analyst on the quality of data
obtained [4]
Concentration Day (ppm)
I 1 329
2 333
I 3 345 4 326 5 322
Polpulation
Mean 331
Std I Deviation 9
b) A chemistry student needs 250 mL of a solution buffered at a pH of 1100 How many grams of ammonium chloride have to be added to 250 mL of 02 mollL NH3 to make such a buffer (Volume is assumed not to change) Kb 18 x 10-5
bull [5]
c) This question is about a buffer solution made by mixing together solutions containing ethanoic acid and sodium ethanoate
i) If you add a small amount of an acid such as dilute hydrochloric acid to this solution the pH doesnt change much Explain what happens to the extra hydrogen ions you have added [21
ii) If you add a small amount of an alkali such as sodium hydroxide solution to the buffer solution again the pH doesnt change much Explain what happens to the extra hydroxide ions you have added [2]
d) The concept of CRM and or SRM is widely used by industry for their quality control measures Briefly explain
i) What are CRM or SRMs [I]
6
ii) What is their central role in analytical chemistry [2]
iii) How are they certified [2]
e) Distinguish between the following
i) Systematic and random errors [2]
ii) Precision and accuracy [2]
7
APPENDIX
Useful Formulas
8
TABLES
TABLE 1 Table of Acid and Base Strength
I Ka Acid Coniu~ate Base Name Formula Formula Name
Large Perchloric acid HCI04 CI04 Perchlorate ion
i 32 10 Hydroiodic acid HI 1shy Iodide 10 10) Hydrobromic acid HBr Br- Bromide I 13 10