Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST p.1 SHAU KEI WAN GOVERNMENT SECONDARY SCHOOL 筲箕灣官立中學 TEST THE TEST Group Members: LAM YAT LONG, ALEX 林日朗 LEUNG TSUM TONG, THOMAS 梁雋堂 WONG TING HEI, SHERMAN 黃庭熙 YIK KAI HEI, ANDREW 易啟希 Teacher Adviser: Mr Lau Chun-On Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) Presentation material: photoalbum.ppt in CD
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Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.1
SHAU KEI WAN GOVERNMENT
SECONDARY SCHOOL
筲箕灣官立中學
TEST THE TEST
Group Members:
LAM YAT LONG, ALEX 林日朗
LEUNG TSUM TONG, THOMAS 梁雋堂
WONG TING HEI, SHERMAN 黃庭熙
YIK KAI HEI, ANDREW易啟希
Teacher Adviser:
Mr Lau Chun-On
P.1
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014)
Presentation material: photoalbum.ppt in CD
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.2
Abstract:
To determine the concentration of sodium hypochlorite, iodometric method is used
currently. However, is it the only method? The goal of this study is to determine
whether two new-designed tests can be possible substitutes of the traditional test.
In the traditional method, chlorine is collected through
iodide solution. The resultant solution is then titrated
against sodium thiosulphate. The concentration of
sodium hypochlorite can be found quite accurately. In
our studies, this tradition test will be used as a reference
to compare the feasibilities of the proposed tests.
In the traditional methods, multiple steps involve and it
leads to errors. So, our first proposed test is a
gravimetric test which only based on one reaction, the
redox reaction between hydrogen peroxide and sodium
hypochlorite. Our 1st test can achieve a test result close
to that from the traditional method.
Our second proposed test is a redox back titration which
requires the addition of iron(II) sulphate and followed
by the titration with potassium dichromate. This
proposed test has similar numbers of steps to the
original one with the use of different reagents.
According to the experimental result, this test has a
larger % difference than our 1st proposed test.
SHAU KEI WAN GOVERNMENT SECONDARY SCHOOL
LAM YAT LONG, ALEX LEUNG TSUM TONG, THOMAS
WONG TING HEI, SHERMAN YIK KAI HEI, ANDREW
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.3
Contents
Introduction P. 4
Principles and theories P. 5-8
Experiments P. 9-22
Data Analysis P. 23-33
Discussion P. 34-37
Further study P. 38-39
Conclusion P. 40
Reference P. 41
Acknowledgement P. 42
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p.4
Introduction:
Normally, to determine the sodium hypochlorite (NaClO) content in bleach, chemists
test for the amount of chlorine in the hypochlorite compound with acid, potassium
iodide (KI) and sodium thiosulphate(Na2S2O3) However, the test is actually quite
complicated. So, is there any simpler yet accurate quantitative test for sodium
hypochlorite?
Our experiment is designed to investigate whether a gravimetric analytical test(with
hydrogen peroxide),and a redox test (with iron(II) sulphate, potassium dichromate)
can test the concentration of sodium hypochlorite accurately and replace the
traditional test. In the experiment, the traditional test will be carried out, which the
result will act as a benchmark for comparison of the accuracy of the two new
designed tests.
Objectives:
1. Determine the sodium hypochlorite concentration in bleach using the
traditional test.
2. Test for the feasibilities of using the two new-designed tests, i.e. the
gravimetric analytical test with hydrogen peroxide and the redox test (with
iron(II) sulphate, potassium dichromate), to determine the sodium
hypochlorite concentration in bleach.
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Principles and theories:
(1) Principles and theories of the traditional test
Determine the sodium hypochlorite concentration with sulphuric acid, potassium
iodide, sodium thiosulphate and starch
The amount of sodium hypochlorite(NaClO) ingredient available can be determined
by allowing a certain quantity of it to react with excess acidified potassium iodide(KI)
solution, the liberated iodine(the amount is determined by the concentration of sodium
hypochlorite), is titrated against the standardized sodium thiosulphate(Na2S2O3)
solution.
When hypochlorite ions mix with excess acidified potassium iodide solution, aqueous
iodine is produced:
OCl- + 2I
- + 2H
+ I2 + H2O + Cl
-.
Aqueous iodine reacts with thiosulphate ions to form iodide ions:
I2 + 2 S2O32-
S4O62-
+ 2 I-
, which the amount of iodide ions can help finding the amount of hypochlorite ions.
Starch is used as indicator for the titration and is added when the solution in the
conical flask turns pale yellow. When iodine is present, the starch-iodine complex
turns the solution dark blue. But the addition of excess sodium thiosulphate solution
will decolorize the solution, as the thiosulphate ions reduce all the iodine back into
iodide ions. So, the first drop of sodium thiosulphate solution causing decolorization
marks the end point of the titration.
Since sodium thiosulphate cannot be a primary standard, standardization is needed to
be performing simultaneously with this traditional test. It can be standardized with
known amount of iodine(I2), which is produced by the redox reaction of known
amount of potassium iodate(KIO3) and excess acidified potassium iodide solution.
When potassium iodate is mixed with acidified potassium iodide, iodine and water is
produced:
6H+
+ IO3- + 5 I
- 3I2 + 3H2O .
Known amount of potassium iodate will be added as the limited reagent, while the KI
solution and dilute sulphuric acid added will be in excess amount. So known amount
of iodine is produced, and is titrated against sodium thiosulphate solution, with starch
solution as the indicator. Every molecule of aqueous iodine reacts with two
thiosulphate ions to form iodide ions:
I2 + 2 S2O32-
S4O62-
+ 2 I- .
When iodine is present, the starch-iodine complex turns the solution dark blue. But
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.6
the addition of sodium thiosulphate solution will decolourize the solution, as the
thiosulphate ions reduce the iodine back into iodide ions. The decolorization of the
solution marks the end point of the titration. Therefore, knowing the amount of iodine
presence and the volume of sodium thiosulphate solution added, the molarity of the
sodium thiosulphate solution can be calculated.
(2) Principles and theories of our proposed tests
TEST 1 Gravimetric analytical method
Determine the sodium hypochlorite concentration with hydrogen peroxide
Sodium hypochlorite is a common oxidizing agent which tends to accept two
electrons in alkaline medium to produce sodium chloride.
The half equation goes like this:
NaClO + H2O + 2e- 2OH
- + NaCl (E°=+0.90).
When hydrogen peroxide(H2O2) is mixed with sodium hypochlorite, redox reaction
occurs. Hydrogen peroxide acts as the reducing agent, and is oxidized to release
oxygen gas. The half equation goes like this:
H2O2 O2 + 2H+ + 2e
- (E°=-0.70).
The overall equation is:
NaClO(aq) + H2O2(aq) H2O(l) + NaCl(aq) + O2(g)
As a result, when excess hydrogen peroxide is added to bleach sample (sodium
hypochlorite solution), oxygen gas is released, that is, the mass of the resultant
solution decreases. Therefore, by calculating the weight difference of the solution, the
amount of oxygen gas released can be found, so does the amount of sodium
hypochlorite in the solution.
TEST 2 Redox back titration
Determine the sodium hypochlorite concentration with iron(II) sulphate, acidified
potassium dichromate and diphenylamine
Sodium hypochlorite is a common oxidizing agent which tends to accept two
electrons in alkaline medium to produce sodium chloride. The half equation goes like
this:
NaClO + H2O + 2e- 2OH
- + NaCl (E°=+0.90).
While iron(II) ion (Fe2+
), in this case, acts as a reducing agent, which lose electrons to
form iron(III) ions(Fe3+
). The half equation goes like this:
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.7
Fe2+
Fe3+
+ e- (E°=-0.77).
When sodium hypochlorite is mixed with iron(II) sulphate(FeSO4) at an alkaline
medium, redox reaction occurs with a standard voltage of +0.13V, which iron(II) ions
will be oxidized to iron(III) ions, and sodium hypochlorite will be reduced to sodium
chloride.
The overall chemical equation:
NaClO(aq) + H2O(l) + 2FeSO4(aq) NaCl(aq) + 2Fe3+
(aq/s)# +
2OH-(aq/s)* + 2SO4
2-(aq)
# The state is either aqueous or solid due to formation of both iron(III) chloride and iron(III)
hydroxide.
* The state is either aqueous or solid due to formation of both sodium hydroxide and iron(III)
hydroxide.
As a result, when excess iron(II) sulphate of known concentration is added to bleach
sample (sodium hypochlorite solution), iron(III) ions are produced. If the amount of
iron(III) ions is found, the amount of sodium hypochlorite in the solution can be
known. Since excess known concentration of iron(II) sulphate is added, finding out
the iron(II) ions remained can let us calculate the amount of iron(III) ions. Therefore,
the next step is to find out the amount of iron(II) ions by back titration with acidified
potassium dichromate.
Potassium dichromate(K2Cr2O7) is a common oxidizing agent in the school laboratory,
which tends to accept six electrons in acidic medium to produce chromium(III)
ions(Cr3+)
.
The half equation goes like this:
14H+ + 6e
- + Cr2O7
2- 2Cr
3+ + 7H2O (E°=+1.33).
While iron(II) ion is a reducing agent, which lose electron to form iron(III) ion. The
half equation goes like this:
Fe2+
Fe3+
+ e- (E°=-0.77).
It will be oxidized to iron(III) ions when it is titrated against acidified potassium
dichromate solution, with a standard voltage of +0.56V. Potassium dichromate is
orange in color, but when it is added to the solution containing iron(II) ions, it
changes into green due to the presence of chromium(III) ion, produced in the
reduction of dichromate ions.
The chemical equation is:
14H+(aq) + Cr2O7
2-(aq) + 6Fe
2+(aq) 2Cr
3+(aq) + 6Fe
3+(aq) + 7H2O(l).
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p.8
However, the colour change from orange to green of dichromate is not sharp for end
point detection. Diphenylamine is added as a redox indicator, with the chemical
formula: (C6H5)2NH, which turns purple when being oxidized.
The equation is 2(C6H5)2 24H20N2 + 2H+ + 2e- :
2 +2H++ 2e
-
It is used as redox indicator in this back titration. It will become oxidized (turns
purple) immediately by acidified potassium dichromate when acidified potassium
dichromate solution is added during the titration. But soon become colorless again
due the reduction by iron(II) ions. When the first drop of excess acidified potassium
dichromate is added, intense color is produced due to oxidation of diphenylamine, and
this signals the end point of the titration.
Therefore, by calculating the amount of acidified potassium dichromate added, the
amount of iron(II) ions can be known and so as the iron (III) ions produced by sodium
hypochlorite. So the amount of sodium hypochlorite can be found.
However, iron(II) sulphate cannot be a primary standard due to it reaction with
oxygen in air. So standardization is needed to be performing simultaneously with this
test. It can be standardized by titration with known amount of acidified potassium
dichromate solution, with diphenylamine as redox indicator. The titration principle is
actually same as the redox titration above.
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p.9
Experiments:
(1) Traditional Test
Test for the amount of ClO- ions with Potassium iodide solution KI(aq), Sodium
thiosulphate solution Na2S2O3(s) by the following equations:
NaClO(aq) + 2HCl(aq) NaCl(s) + Cl2(g) + H2O(l)
Cl2(g) + 2KI(aq) 2KCl(aq) + I2(aq)
I2(aq) +2Na2S2O3(aq) 2NaI(aq) +Na2S4O6(aq)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Part 1 Standardization of sodium thiosulphate solution Na2S2O3(aq) by the following
equation:
IO3- + 6H
+ + 5I
- 3I2 + 3 H2O
I2 + 2 S2O32-
S4O62-
+ 2 I-
Reagents:
~2M H2SO4solution
KI solid
NaIO3 solid
~0.01M Na2S2O3 solution
Starch solution
Apparatus:
100.0 cm3 and 250.0 cm
3 volumetric flasks
Burette
Magnetic stirrer
Conical flasks
Beakers
Electronic balance
25.0 cm3 pipettes
Stand and clamps
Procedures:
Preparation of KI solution
1. Using an electronic balance, weigh about 14 g of KI(s)
2. Dissolve the KI(s) in a beaker containing about 150 cm3
of distilled water
3. Pour all the KI solution into a 250.0 cm3 volumetric flask.
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
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4. Add distilled water until the meniscus reaches the graduation mark.
5. Stopper the flask and turn it upside down for several times.
Preparation of NaIO3 solution
1. Using an electronic balance, weigh about 2 g of NaIO3(s), record the mass
weighted.
2. Dissolve all the NaIO3(s) in a beaker containing about 150 cm3
of distilled water
3. Pour all the NaIO3 solution into a 250.0 cm3 volumetric flask.
4. Wash the inner of the beaker and pour all the washing into the volumetric flask for
several times.
5. Add distilled water until the meniscus reaches the graduation mark.
6. Stopper the flask and turn it upside down for several times.
7. Using a 25.0 cm3 pipette, collect 25.0 cm
3 of the NaIO3 solution from the
250.0 cm3 volumetric flask to a 100.0 cm
3 volumetric flask for dilution.
8. Add distilled water until the meniscus reaches the graduation mark.
9. Stopper the flask and turn it upside down for several times.
Titration
1. Using a measuring cylinder, collect about 15 cm3 of KI solution to a conical flask.
2. Using a 25.0 cm3 pipette, collect 25.0 cm
3 of the NaIO3 solution to the conical flask
3. Using a measuring cylinder, collect about 5cm3 of the H2SO4 solution to the conical
flask.
4. The color of the solution in the conical flask should have turned from colorless to
deep brown.
5. Place the conical flask on a magnetic stirrer, turn on the stirrer.
6. Titrate the solution in the conical flask against the Na2S2O3 solution, record the
initial reading of the burette.
7. When the color of the solution in the conical flask turns to pale yellow, add starch
as indicator, the color of the solution should turn into deep blue
8. Titrate until the color of the solution in the conical flask turns from deep blue to
colorless.
9. Record the final reading
10. Repeat the steps and perform the titration for four times.
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Part 2
Test for the amount of NaClO(aq) with Potassium iodide solution KI(aq), Sodium
thiosulphate solution Na2S2O3(aq) by the following equations:
NaClO(aq) + 2HCl(aq) NaCl(s) + Cl2(g) + H2O(l)
Cl2(g) + 2KI(aq) 2KCl(aq) + I2(aq)
I2(aq) + 2Na2S2O3(aq) 2NaI(aq) + Na2S4O6(aq)
Experimental Set-up:
Explanation of the set-up:
Acidified KI solution will be transferred to the filtering flask that contains diluted
bleach solution by the syringe so that all Cl2 gas produced can immediately react with
the surrounding KI solution, which prevent the leakage of Cl2 gas which may make
the result of the test under-estimated. The closed system is to ensure that although
there may be some Cl2 gas that cannot react with the KI solution fast enough, it can
still be transferred to the boiling tube containing KI solution and react with the KI
solution, which can minimize the error of the test.
Reagents:
Bleach (NaClO solution)
~2M HCl solution
KI solid
~0.07M standardized Na2S2O3 solution
Starch solution
Fig.1
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Apparatus:
100 cm3 syringe
Delivery tubes
Stopper with a hole
Stopper with two holes
Burette
Magnetic stirrer
Conical flasks
Boiling tubes
25.0 cm3 pipettes
Stand and clamps
250.0 cm3 volumetric flasks
Procedures:
1. Using an electronic balance, weight about 45 g of KI(s).
2. Dissolve the KI(s) in a beaker containing about 150 cm3
of distilled water.
3. Pour all the KI solution into a 250.0 cm3 volumetric flask.
4. Add distilled water until the meniscus reaches the graduation mark.
5. Stopper the flask and turn it upside down for several times.
6. Use a pipette to transfer 25.0 cm3of the bleach (NaClO solution) to another
250.0 cm3 volumetric flask.
7. Add water into the volumetric flask until the meniscus reaches the graduation mark.
8. Prepare the set up (see Fig. 1).
9. Use a pipette to transfer 25.0 cm3of the diluted NaClO solution from the volumetric
flask to the filtering flask of the set up.
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.13
10. Add about 30 cm3 1M KI solution into the boiling tubes of the set up.
11. Stopper the filtering flask and the boiling tube of the set up.
12. Mix about 50cm3 1M KI solution with ~50cm
3 2M HCl in a beaker.
13. Using the syringe, collect the KI solution prepared in step 12, connect the rubber
tubing and add the solution to the filtering flask containing bleach of the set up
slowly.
14. The color of the solution in the filtering flask should become reddish brown in
color.
15. Collect all the solution in a 250.0 cm3 volumetric flask, use distilled water to wash
the container and pour all the washing into the 250.0 cm3 volumetric flask.
16. Add water into the volumetric flask until the meniscus reaches the
graduation mark.
17. Use a 25.0 cm3 pipette twice to collect 50.0 cm
3 of the solution from the
volumetric flask to a small conical flask.
18. Place the conical flask on a magnetic stirrer, turn on the stirrer.
19. Titrate the solution in the conical flask against the standardized Na2S2O3 solution,
record the initial reading of the burette.
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.14
20. When the color of the solution in the conical flask turns to pale yellow, add starch
as indicator, the color of the solution should turn into deep blue.
21. Titrate until the color of the solution in the conical flask turns from deep blue to
colorless.
22. Record the final reading.
23. Repeat the steps and perform the titration for four times.
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.15
(2) Proposed test
TEST 1 Gravimetric analytical method
Determine the sodium hypochlorite concentration with hydrogen peroxide
Gravimetric Analysis of ClO¯ ion with H₂O₂ solution by the following equation:
NaClO(aq) + H2O2(aq) NaCl(aq) + O2(g) + H2O(l)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Experimental Set-up:
Apparatus:
two electrical balances
beakers
25.0 cm3
pipette
Reagents:
Bleach (NaClO solution)
~ 0.5M Hydrogen Peroxide solution
Procedures:
1. Set up the apparatus (refer to Fig.2) .
2. Weigh an empty clean beaker with an electrical balance. Then, transfer 25.0 cm3
bleach solution to the beaker that put on the electrical balance, record the weight.
3. Pour 100 cm3 hydrogen peroxide into another beaker.
Fig. 2
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.16
4. Put the hydrogen peroxide-containing beaker on the electrical balance, record the
reading.
5. Add the hydrogen peroxide solution to the bleach-containing beaker on the
electrical balance, with a glass rod guiding the liquid.
6. Weigh the emptied hydrogen peroxide-containing beaker with an electrical
balance after pouring out the hydrogen peroxide.
7. Record the mass of the mixture every 30 seconds in the first 20 minutes.
8. After 20 minutes, record the weight of the mixture-containing beaker every five
minutes
until a steady reading is received.
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.17
TEST 2 Redox back titration
Determine the sodium hypochlorite concentration with iron(II) sulphate, acidified
potassium dichromate and diphenylamine
Redox Back-Titration of NaClO with FeSO4, acidified K2Cr2O7 and (C6N5)2NH by the
following reactions:
1. Reduction of NaClO with FeSO4
NaClO(aq) + 2FeSO4(aq) + H2O(l)
NaCl(aq) + 2Fe3+
(aq) + 2SO42+
(aq) + 2OH-(aq)
2. Oxidation of Fe2+
ion with K2Cr2O7
14H+(aq)
+ K2Cr2O7(aq) + 6FeSO4(aq)
2Cr3+
(aq) + 6Fe
3+(aq)
+ 6SO4
2-(aq) + 2K
+(aq) + 7H2O(l)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Experimental Set-up:
Fig. 3
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p.18
Apparatus Required:
250 cm3 conical flasks
a 50 cm3 burette
a stand and burette clamp
10.0 cm3 and 25.0 cm
3pipettes
a 250.0 cm3 volumetric flask
Beakers
Reagent Required:
Household bleach( same bottle as for other tests)
FeSO4 solution,
i.e. dissolving about 20g FeSO4 •7H2O into approximately 500 cm3 water
~0.014M K2Cr2O solution(acidified),
i.e. dissolving about 1g anhydrous K2Cr2O7 into a 250.0 cm3 volumetric flask with
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.19
~100 cm3 deionized water and ~150 cm
3 2M H2SO4 solution
Diphenylamine (as a redox indicator, melted in hot concentrated sulphuric acid)
Procedure:
Part 1: Standardization of FeSO4
1. Setup the apparatus (refer to Part 1 of Fig.3) .
2. Transfer 18.0 cm3 FeSO4 into the conical flask.
3. Add a few drops of redox indicator into the solution .
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.20
4. Record the initial reading of the acidified K2Cr2O7 solution.
5. Titrate the mixture against the acidified K2Cr2O7 until the solution just change
from green to dark purple.
6. Record the amount of acidified K2Cr2O7 solution added.
7. Repeat the procedure at least three times to obtain an accurate data of titrant
required. Repeat the procedures 1-7 with 10.0 cm3 FeSO4 used in step 2.
Part 2: Reduction of NaClO with FeSO4
1. Wash a 25.0 cm3 pipette first with distilled water and then with FeSO4.
Wash a 25.0 cm3 pipette first with distilled water and then with bleach.
2. Transfer 25.0 cm3 bleach solution into a 250.0 cm
3 volumetric flask.
3. Add distilled water until the meniscus reaches the graduation mark.
4. Stopper the flask and turn it upside down for several times.
5. Transfer 3.4 cm3
diluted bleach into a conical flask using a 10.0 cm3 pipette that
was washed with the diluted bleach.
6. Transfer 25 cm3 FeSO4 Solution into the same conical flask.
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.21
7. Wait 5 minutes for reaction to occur until red precipitate formed (i.e. Fe (OH)3).
Part 3: Oxidation of Fe2+
ion with K2Cr2O7 solution:
1. Wash the burette first with deionized water, subsequently with acidified K2Cr2O7
solution.
2. Set up apparatus (refer to the part 3 of Fig.3)
3. Place the conical flask of resultant mixture under the burette and add a small
amount of redox indicator into the mixture
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
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4. Record the initial reading of the acidified K2Cr2O7 solution
5. Titrate the mixture against the acidified K2Cr2O7 until the mixture just change
from green to dark purple .
6. Record the amount of acidified K2Cr2O7 solution added
7. Repeat the procedure at least three times to obtain an accurate data of titrant
required
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.23
Data and data analysis:
(1) Traditional test
Determine the sodium hypochlorite concentration with sulphuric acid, potassium
iodide, sodium thiosulphate and starch
Equation involves:
NaClO(aq) + 2HCl(aq) NaCl(s) + Cl2(g) + H2O(l)
Cl2(g) + 2KI(aq) 2KCl(aq) + I2(aq)
I2(aq) + 2Na2S2O3(aq) 2NaI(aq) + Na2S4O6(aq)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Part 1 Standardization of sodium thiosulphate solution Na2S2O3(aq) by the following
equation:
IO3- + 6H
+ + 5I
- 3I2 + 3 H2O
I2 + 2 S2O32-
S4O62-
+ 2 I-
Mass of NaIO3(s) used /g 2.078
Number of mole of NaIO3= Mass of NaIO3(s) used / Molar mass of NaIO3(s)
= (2.078) / (197.8924)
=0.010500655mol
All NaIO3(s) is dissolved in 250.0 cm3 solution.
Molarity of the NaIO3 solution = Number of mole of NaIO3 / Volume
= (0.010500655) / (250/1000)
= 0.042002623 mol dm-3
25.0 cm3 of the NaIO3 solution is diluted to 100.0 cm
3
Number of mole of NaIO3 in the diluted solution=Molarity of the initial NaIO3
solution × Volume used
=(0.042002623) × (25/1000)
=1.050065591×10-3
mol
Molarity of the diluted NaIO3 solution= Number of mole of NaIO3 used for dilution /
Volume
= (1.050065591×10-3
) / (100/1000)
=0.010500655 mol dm-3
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.24
25.0 cm3 of the diluted solution is used for titration
Number of mole of NaIO3 used = Molarity of the diluted NaIO3 solution × Volume
used
= (0.010500655) × (25/1000)
= 2.625163978×10-4
mol
Titration data
Volume of Na2S2O3 used Trial 1st
2nd
3rd
Initial reading /cm3
1.60 23.50 3.70 12.40
Final reading /cm3
23.50 45.40 25.60 34.30
Difference /cm3
21.90 21.90 21.90 21.90
By IO3- + 6H
+ + 5I
- 3I2 + 3 H2O
Number of mole of I2 formed = Number of mole of NaIO3 × 3
= 2.625163978×10-4
× 3
=7.875491934×10-4
mol
By I2 + 2 S2O32-
S4O62-
+ 2 I-
Number of mole of Na2S2O3 reacted = Number of mole of I2×2
= 7.875491934×10-4
×2
=1.575098387×10-3
mol
Molarity of Na2S2O3 solution = Number of mole of Na2S2O3 / volume used in titration
= (1.575098387×10-3
) / (21.90/1000)
= 0.0719223 mol dm-3
Molarity of Na2S2O3 solution: 0.0719223 mol dm-3
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST
p.25
Part 2
Test for the amount of NaClO(aq) with Potassium iodide solution KI(aq), Sodium
thiosulphate solution Na2S2O3(aq) by the following equations:
NaClO(aq) + 2HCl(aq) NaCl(s) + Cl2 (g) + H2O(l)
Cl2(g) + 2KI(aq) 2KCl(aq) + I2(aq)
I2(aq) + 2Na2S2O3(aq) 2NaI(aq) + Na2S4O6(aq)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Titration data
Volume of Na2S2O3 used Trial 1st
2nd
3rd
Initial reading /cm3
1.50 10.10 18.70 27.30
Final reading /cm3
10.10 18.70 27.30 35.90
Difference /cm3
8.60 8.60 8.60 8.60
Reagent data
Molarity of Na2S2O3 solution: 0.0719223 mol dm-3
Number of mole of Na2S2O3 used in titration = Volume used in titration × Molarity of
Na2S2O3 solution
= (8.60 / 1000) × (0.0719223)
= 6.185317866 × 10-4
mol
Let MNaClO be the molarity of NaClO in bleach.
25 cm3 of bleach solution is collected and diluted to 250 cm
3
Number of mole of NaClO in the diluted bleach solution = molarity of NaClO in
bleach × Volume used
= MNaClO × (25/1000)
= 0.025 MNaClO
Molarity of NaClO in the diluted bleach solution = Number of mole of NaClO /
Volume
= 0.025 MNaClO / (250/1000)
= 0.1 MNaClO
25 cm3 of the diluted solution is used for reaction
Number of mole of NaClO reacted = Molarity of NaClO in the diluted bleach solution
× Volume used
= 0.1 MNaClO × (25/1000)
= 0.0025 MNaClO
Hong Kong Chemistry Olympiad for Secondary Schools (2013 - 2014) TEST THE TEST