Synthesis and Analysis of a Cobalt Coordination Complex

Synthesis and Analysis of a Cobalt Coordination Complex

Co Some of the most interesting area in inorganic chemistry has centered on the preparation and properties of coordination compounds. These compounds, sometimes called complexes, are salts that contain complex ions. A complex ion contains a central metal atom to which are bonded small molecules or ions, called ligands. When the metal atom is a transition metal the complex ion is usually colored (resulting in the absorption of visible light) and often contains unpaired electrons giving rise to their paramagnetic properties. The compound we will prepare contains cobalt ion, ammonia, and chloride ions. Its formula is of the form Cox(NH3)yClz. After we complete the synthesis of this compound we will systematically analyze for each component of the compound. The experiment is done over four laboratory periods. Complete advanced study assignment for each part before coming to lab! Part 1. Synthesis of Cox(NH3)yClz

Reagents CoCl2·6H2O ⎯⎯⎯→ Cox(NH3)yClz 1. (Two students (partners) will work together and share data). Set up in the fume hood a clean dry

125-mL Erlenmeyer flask and place it on a ceramic hot plate with a small magnetic stirrer bar. 2. In the balance room tare plastic boat (set centigram balance to 0.00 g) and mass out approximately

(~) 5 g (±0.1g) of ammonium chloride. Transfer all of this salt to the Erlenmeyer flask. 3. Located in another fume hood, obtain ~ 30 mL of 15 M aqueous ammonia, NH3, in a 50 mL beaker

and cover it with a small watchglass and go back to your reaction flask. CAUTION: This is a concentrated basic reagent, with a strong pungent odor that can knock you unconscious.

4. While under the fume hood, remove the watchglass and carefully add 30 mL of 15 M NH3 to the Erlenmeyer flask. You can do this with your plastic pipet if you are hesitant in pouring from the 50 mL beaker. [The combination of NH4Cl and NH3(aq) guarantees a large excess of the ligand, NH3.] Turn on the magnetic stirrer (left knob on your stirrer/hot plate) and allow the NH4Cl to completely dissolve.

5. Using a tared plastic boat mass out 10 g (to the nearest ± 0.01 g) of finely divided CoCl2·6H2O. Record this in your notebook.

6. While stirring the ammonium chloride - ammonia solution add the CoCl2·6H2O in small portions to your reaction flask. This can be accomplished simply by holding the plastic boat near the mouth of your reaction flask. Use your Ni-spatula and scrape a small amount of the purple solid into the reaction mixture. Allow solid to dissolve and continue adding in small portions until all has been added.

7. Go back to the fume hood containing the hydrogen peroxide, H2O2, reagent. Dispense ~ 8 mL of 30% H2O2 into your 50 mL beaker and cover with a small watchglass. {No need to rinse beaker since by this time all of the NH3 you obtained has evaporated.}

8. Back at your reaction flask use your plastic pipet and SLOWLY add in small portions down the inside wall of your reaction flask the 8 mL of 30% H2O2 to the brown slurry. [CAUTION: 30% hydrogen peroxide is a strong oxidizing agent that can cause severe burns and bleaching of skin and clothing. Use gloves when handling it.] Avoid excessive effervescence (gas evolution) in this very exothermic reaction. [If the addition is SLOW and the reaction still shows excessive effervescence, turn off the magnetic stirrer momentarily or remove the reaction flask from the stirrer.] The initial Co2+ ion has now been oxidized to Co3+ ion that is in the form of a complex ion.

SACC page 2 9. Obtain ~30 mL of 12 M HCl solution in your 50 mL beaker {use the same 50 mL beaker as before}

and cover with watchglass while transporting it to your reaction mixture. When the bubbling stops, with the use of your plastic pipet SLOWLY add in small portions down the inside wall of your reaction flask 30 mL of concentrated (12 M) HCl. [CAUTION: Concentrated HCl is a strong, fuming acid that will cause severe burns and breathing difficulties, so handle with care!] The white cloud that forms is solid ammonium chloride, formed by a gaseous acid-base neutralization reaction, dispersed in air.

10. After the addition of the acid has been completed, the reaction may be removed from the fume hood. Use a hot plate with stirrer; place the reaction solution in a water bath (about 75 mL of H2O in 250 or 400 mL beaker and heat the solution to 70˚C with occasional stirring (leave stirrer on). For best results keep the temperature between 60˚C and 75˚C for 15-20 minutes after obtaining reaction temperature.

11. Remove reaction flask from water bath, add ~25 mL deionized water, (request instructor to remove your magnetic stirrer bar!) and allow the solution to cool to ~10˚C in an ice-water bath (a 400 mL beaker containing ~75 mL of water plus ice). Precipitation of the purple product should occur. Cool ~ 20 mL of deionized (dH2O) water in your ice-water bath after removing reaction mixture.

12. Collect the product by filtration through a Buchner funnel. Seat a #1 filter paper by running a little distilled water through the Buchner funnel with the vacuum on, as demonstrate by your lab instructor. A single circular #1 filter paper should fit over the holes of the Buchner funnel. Wash the product three times with a total of 15 mL of cold deionized water and at least twice (smaller portions better) with a total of 15 mL ice cold 95% ethanol. Allow the aspirator to draw air through the precipitate for 5-10 minutes to further dry the product. Transfer the product to a LARGE watch glass; spread the product out with your spatula. The consistency of your product should be like dry sand, the product does not stick to the spatula. Loosely cover with a paper towel, and allow drying until the following laboratory period.

SACC page 3 Part 2. Gravimetric analysis of Chloride in Cox(NH3)yClz Here we will determine how many chloride ions are bound to the metal as ligands (part of the metal coordination sphere) and how many serve as counter ions which are not covalently bound to the metal. 1. Mass the dry purple product that you synthesized to the nearest ± 0.01 g. Record in your notebook

(Partners share this data!) {First we find how many chlorides are not attached to the metal (unheated sample!), then we repeat steps 2 and 3 and go to step 6 (heated sample!)}

2. Each member of the team now works individually, the data obtained will be shared and this will account for two (duplicate) determinations! Using an analytical balance, mass out a sample of the purple product in a “weighing” boat between 0.45 – 0.55 g and record this mass to the nearest 0.1 mg (±0.0001g). Transfer this to a 250 mL beaker.

3. Obtain a magnetic stirrer/heater hot plate, place the 250 mL beaker on the hot plate and add a magnetic stirring bar (~ 1 in long). Add ~150 mL dH2O and stir to completely dissolve compound. Acidify this solution by adding ~1-1.5 mL of 6M nitric acid (HNO3) and continue stirring. {If this is the Heated sample go to Step 6! Otherwise continue below.}

4. Obtain ~30mL of 0.25 M AgNO3 solution and add it in small quantities to your reaction mixture with your plastic pipet. Stir reaction mixture for about 5 minutes after you have completed the AgNO3 addition. While the mixture is stirring obtain a clean, dry fritted glass crucible from the oven or your instructor. Use a pair of crucible tongs and a wire gauze held underneath as demonstrated by your instructor. Mass the glass crucible on the analytical balance to ±0.0001 g. Record this in your notebook (mass crucible 1{No heating}).

5. Cool reaction mixture to ~10˚C in an ice-water bath (100 mL of ice/H2O in 400 mL beaker). Also cool about 15 mL of 0.01 M HNO3 for washing. Go to step 9.

6. Obtain ~30mL of 0.25 M AgNO3 solution and add it in small quantities to your reaction mixture with your plastic pipet. Heat the suspension with stirring until the solution boils. Continue boiling the mixture with stirring for ~20 minutes (but see step 8). This “digestions” process aids the completeness of precipitation and coagulation of the solid AgCl.

7. While the solution is boiling obtain a clean, dry fritted glass crucible from the oven or your instructor. Use a pair of crucible tongs and a wire gauze held underneath as demonstrated by your instructor. Mass the glass crucible on the analytical balance to ±0.0001 g. Record this in your notebook.

8. After the color of the solution fades to orange remove the beaker from hot plate, ask instructor to remove stirrer bar, and begin cooling to ~10˚C in an ice-water bath (100 mL of ice/H2O in 400 mL beaker). Also cool about 15 mL of 0.01 M HNO3 for washing.

9. Set up glass crucible, neoprene adapter, filter flask apparatus and connect to aspirator. Turn aspirator on full and check for vacuum leaks and for rate of flow. (If water does not come through fast enough, check with instructor, you may have an old glass crucible and needs to be replaced.) Filter solution through glass crucible first, and using your squeeze bottle squirt small portions of water behind precipitate and rinse all of it into the glass crucible.

10. Wash the precipitate with ~15 mL, in small portions, of cold 0.01 M HNO3 until the washing (few drops) collected in a test tube gives no turbidity when tested with a drop of 3M HCl. The turbidity can be tested by breaking the vacuum above the neoprene adapter, removing the glass crucible and transferring a few drops of the solution coming out the bottom of the crucible to a small test tube, as demonstrated by your instructor.

SACC page 4 11. When the precipitate has been collected wipe the outside of your glass crucible with moist paper

towel and dry to remove oil and fingerprints. Place the glass crucible in a small, dry 50 mL beaker insert a piece of paper with your NAME, heated or unheated sample, and locker number between the crucible and the beaker and place it in the container provided for drying. This completes the chloride analysis if you have repeated steps 2 and 3 twice. Otherwise go back repeat the procedure for the heated sample from step 2 – step 3 and then skip to step 6.

SACC page 5 Part 3. Spectrophotometric Analysis of Cobalt(II) ion in Cox(NH3)yClz 1. Determine the visible spectrum of Co2+ ion in a 0.075 M CoSO4(aq) stock solution. Record the

percent transmittance, %T (±0.1%T), of the solution every 20 nm from ~360-700 nm. Spectrophotometer must be recalibrated between each wavelength measurement. Convert your %T values to Absorbance =2-log(%T) and determine the wavelength of maximum absorbance. All subsequent samples will be read at this wavelength. Use Excel or similar program and graph the absorbance versus wavelength to be included as part of your report.

2. Prepare six standard solutions of cobalt(II) ion by diluting the 0.075 M stock solution (above) with deionized water using a 10-mL graduated pipet. Make the dilutions according to the table below: Std# mL 0.075 M stock solution mL H2O 1 5.00 5.00 2 4.00 6.00 3 3.00 7.00 4 2.00 8.00 5 1.00 9.00 6 0.90 9.10 7 Blank Few mL water Cover standard solution with parafilm to prevent evaporation. After you prepare your unknown purple sample for Co-analysis you will read the %T of all solution consecutively.

3. Mass out 1-1.2 g of the unknown purple compound to (±0.0001 g) on the analytical balance into a 50-mL beaker. Record this mass in your notebook. Cover the beaker with a watch glass, place it on a hot plate in the fume hood and heat until the solid sample liquefies, foams and turns blue. This process frees the cobalt ion from the other ligands in the complex. Remove the beaker from the hot plate and allow it to cool to room temperature (r.t.). Add 10 mL of deionized water and 1 mL of concentrated sulfuric acid to the sample. [CAUTION: Concentrated H2SO4 is a strong acid that will cause severe burns and may damage clothing, so handle with care!] Dissolve any solid that remains by boiling the mixture gently on a hot plate. Cool the solution to r.t. Quantitatively transfer the contents of the beaker to a clean 100.0 mL volumetric flask. This is accomplished by rinsing the beaker (after careful transfer of the solution) with small portions (5 mL 2 to 3 times) of deionized water. All the washings are added to the volumetric flask. This will ensure that all of the solution goes into the flask. Stir the solution in the volumetric flask and fill to the mark with deionized water from your wash bottle. Stopper the flask and mix thoroughly (invert it several times). Using the wavelength of maximum absorbance as determined from the visible absorbance spectrum of the cobalt(II) ion stock solution, measure and record the %T (±0.1T%) of all the standard solutions and your unknown cobalt solution from your purple product. Record all data in your notebook.

SACC page 6 Part 4. Thermal decomposition and volumetric analysis of NH3 in Cox(NH3)yClz

You may determine the number of moles of ammonia per mole of your purple Co-complex by decomposing a known mass of your cobalt complex by heating in sodium hydroxide. This liberates NH3 gas, which you can trap as ammonium ion (NH4

+) in a known quantity of standardized HCl. You can determine how much HCl reacted with the ammonia, by back-titration with standard NaOH, and thereby you can determine how many moles of NH3 were liberated from the complex.

Co(NH3)nClm + NaOH ⎯⎯→ Co2O3(s, black) + nNH3(g )

NH4+

(aq) + left over HCl

~H2O

Procedure: Set up the gas delivery apparatus as shown in the photo. The orange plastic clamp goes with your 250 mL Erlenmyer Flask (EF) reaction vessel, and the green plastic clamp goes with the 25 x 250 mm test tube used for the thermal decomposition of your synthesized Co-complex.

Also set up two burets, one for your standardized NaOH and the other for your standardized HCl. The HCl buret can be shared by the entire bench to deliver the required amount of this reagent. Obtain approximately 60 mL of standardized NaOH and ~100 mL (more as needed) of standardized HCl in labeled beakers. Record their exact concentrations in your notebook. Rinse your burets with the appropriate solutions, fill and remove trapped air from the stopcock/buret tip area. Make sure that you fill the buret with the standardized NaOH and not with the ~3M NaOH! Use buret caps for the NaOH buret to prevent CO2 diffusion that will change the concentration. Carefully deliver 30.00 mL of the standardized HCl into the clean 250 mL Erlenmyer Flask (EF) of the gas delivery apparatus (figure) from your HCl buret. Place EF on a magnetic stirrer plate. Add a magnetic stir bar (may be obtained from the instructor. Add ~20 mL of deionized water and 2-3 drops of bromcresol green indicator to the HCl solution in the EF. Record the color. Replace the glass tubing/rubber stopper assembly securely

(tightly) into the neck of the EF. Wrap a rubber band around the plastic cap to secure the rubber stopper.

Xs Std HCl

Reacted with Std NaOH to a bromocresol green endpoint

SACC page 7 Mass out ~0.25 g of your purple complex on the analytical balance (record this mass to the nearest ± 0.0001 g) and put it in the large 25x250 mm test tube of the apparatus. Clamp the test tube about 16 inches above the bench top at a 45˚ angle as shown in the photo. Now add ~3 mL of 3 M NaOH to the complex in the test tube, and put in the stopper attached to the rubber tubing of the gas delivery apparatus. Wrap a rubber band around the plastic cap to secure the rubber stopper. Your apparatus is now charged and sealed. Turn on the magnetic stirrer and stir solution constantly while decomposing your Co-complex and collecting ammonia gas. Ignite your Bunsen Burner and adjust the flame to obtain a blue inner cone. GENTLY (and I have raised my voice here) start heating the test tube by moving the Bunsen Burner from bottom toward the top of the test tube but don’t go beyond the upper end of the test tube because you will burn the rubber stopper. Move the Bunsen Burner flame back and forth several times along the length of the test tube to remove condensed water vapor that has collected in the upper portion of the test tube. You will have to apply this technique periodically to drive the NH3 gas and water vapor into the gas collection flask (EF). As you proceed with the decomposition of the purple complex your goal is to get the solution to come to a gentle rolling boil, drive the ammonia gas and water vapor into the EF and trap it in the solution in the EF. The rolling boil we are looking for is when the heated liquid is producing gentle bubbles consistently. The rolling boil can be achieved by passing the Bunsen Burner flame intermittently (back and forth) across the lower end of the test tube. When the solution comes to a gentle boil you can place the Bunsen burner on the desktop under the bottom of the test tube and adjust the flame to maintain this gentle rolling boil. Allow the magnetic stirrer to continually stir the solution without splashing the solution in the EF. With a rolling boil the hot NaOH solution will decompose the complex, liberating NH3(g). As the magnetic stirrer mixes the solution in the EF the HCl solution will trap the ammonia gas given off as ammonium ions, NH4

+. {Also check the rubber stopper on the EF periodically so it remains tight as you are collecting the ammonia gas.} Following the above directions the gas collection should take approximately 15-20 minutes. The decomposed complex will turn black (Co2O3). Continue stirring the EF and gently heat the test tube to dryness. Record in your notebook color changes of the complex, and of the bromocresol green indicator. When the complex is decomposed completely, wait a few minutes, un-stopper the test tube and with a wash bottle rinse the glass delivery tube into the EF with small amount (~ 5 mL portion=aliquot) of distilled water. Set the Erlenmeyer flask aside to titrate (next paragraph). Clean the used test tube with a test tube brush and soap. Rinse with tap water followed by deionized water and set up for a duplicate ammonia determination.

CAUTION: Once you have established a rolling gentle boil, stir the EF periodically to eliminate the chance of excessive gas pressure and loss of ammonia!

Titration: Record the initial volume in the NaOH buret, titrate the excess HCl in your 250 mL EF until you reach the bromcresol green end point, which is Blue (yellow-green-BLUE). Record the final volume in the buret and calculate the volume of NaOH used. Repeat analysis for a second determination.

Synthesis and Analysis of a Cobalt Coordination Complex

Co Name: ____________________ Partner’s Name: ____________________ Lab Section: MW/TTH/M-TH (circle)

Data: Synthesis and Analysis of a Coordination Compound Part 1. Synthesis of Cox(NH3)yClz Mass of CoCl2·6H2O _______________ g

Mass of purple product (obtained) _______________ g

{Samples of this will be used in subsequent analyses!}

Part 2. Gravimetric determination of Chloride Ion in Cox(NH3)yClz

Mass of sample I (unheated) _______________ g Mass of sample II (unheated) _______________ g

Mass of glass crucible (I) _______________ g Mass of glass crucible (II) _______________ g

Mass(crucible + AgCl) (I) _______________ g Mass(crucible + AgCl) (II) _______________ g

Mass of AgCl (I) _______________ g Mass of AgCl (II) _______________ g

Mass of sample I (heated) _______________ g Mass of sample II (heated) _______________ g

Mass of glass crucible (I) _______________ g Mass of glass crucible (II) _______________ g

Mass(crucible + AgCl) (I) _______________ g Mass(crucible + AgCl) (II) _______________ g

Mass of AgCl (I) _______________ g Mass of AgCl (II) _______________ g

Part 3. Spectrophotometric determination of Cobalt(II) ion in Cox(NH3)yClz

Mass of sample I (your product) _______________ g Mass of sample II (your product) _______________ g

Standard Solution 1: __________ mL of stock solution __________ mL of water %T __________

Standard Solution 2: __________ mL of stock solution __________ mL of water %T __________

Standard Solution 3: __________ mL of stock solution __________ mL of water %T __________

Standard Solution 4: __________ mL of stock solution __________ mL of water %T __________

Standard Solution 5: __________ mL of stock solution __________ mL of water %T __________

Standard Solution 6: __________ mL of stock solution __________ mL of water %T __________

Blank (for calib.) 7: ____0_____ mL of stock solution ____~3____ mL of water %T 100_(manually set)

Wavelength of maximum absorbance = _______________ nm {Obtained from Excel plot of Absorbance v. wavelength}

%T (sample I) __________ %T (sample II) __________

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Part 4. Thermal Decomposition and Volumetric Analysis of NH3 in Cox(NH3)yClz Mass of sample I (your product) _______________ g Mass of sample II (your product) _______________ g

Molarity of standardized HCl __________ M

Molarity of standardized NaOH __________ M 2nd Trial

Initial reading HCl buret __________ mL __________ mL

Final reading HCl buret __________ mL __________ mL

Initial reading NaOH buret __________ mL __________ mL

Final reading NaOH buret __________ mL __________ mL

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Data for Calculations: Synthesis and Analysis of a Coordination Compound

5. Percent yield of Cox(NH3)yClz Molar Mass of CoCl2·6H2O __________ g/mol Show calculations for Mmass of product & %Y here!

Molecular formula of product _______________

(fill in after completion of Part 4!)

Molar mass of product __________ g/mol

Percent yield __________ %

6. Percentage Chloride Ion in Cox(NH3)yClz {From heated reactions Only. Two determinations!}

Mass of sample I (from data P2) _______________ g Mass of sample II (from data P2) _______________ g

Molar Mass of AgCl __________ g/mol Molar Mass Cl __________ g/mol

Mass of AgCl (sample I) __________ g Mass of AgCl (sample II) __________ g

Mass of Cl- (sample I) __________ g Mass of Cl- (sample II) __________ g

Percent Cl- (sample I) __________ % Percent Cl- (sample II) __________ %

Average %Cl(heated) = _____________%

Mass of AgCl (sample I-unheated) __________ g Mass of AgCl (sample II-unheated) __________ g

Mass of Cl- (sample I-unheated) __________ g Mass of Cl- (sample II-unheated) __________ g

Percent Cl- (sample I-unheated) __________ % Percent Cl- (sample II-unheated) __________ %

Average %Cl(unheated) = _____________%

What is the ratio (expressed as integers) of the average %Cl(heated) to the average %Cl(unheated)? ___________

From the integer ratio obtained above,

How many chloride ions are present in your Co compound? ______; How many are coordinated to Co? _________

Briefly Explain how you determined this.

Show calculations for each unique determination below!

SACC page 11

7. Percentage Cobalt(II) ion in Cox(NH3)yClz {Two determinations!}

Mass of sample I (from data P3) _______________ g Mass of sample II (from data P3) _______________ g

The equation of the Least Squares straight line (Excel’s Trendline) from standard solutions:___________________________

{Also attach your Standard Absorbance vs. Concentration output from Excel to your report!}

Molar Mass Co = _____________ g/mol

[Co] (sample I) = _____________M [Co] (sample II) = _____________M

Mass Co (I) = _____________ g Mass Co (II) = _____________ g

% Co (sample I) = ____________ % % Co (sample I) = ____________ %

Average % Co = _____________ % Show calculations for one determination below!

8. Percentage of NH3 in Cox(NH3)yClz {Two determinations!} Mass of sample I (from data P4) _______________ g Mass of sample II (from data P4) _______________ g

Molarity of standardized HCl _____________ M Molarity of standardized NaOH ______________ M

Volume of HCl used (I) ______________mL Volume of HCl used (II) ______________mL

Moles HCl used (I) ______________mol Moles HCl used (II) ______________mol

Volume of NaOH used (I) ______________mL Volume of NaOH used (II) ______________mL

Moles NaOH used (I) ______________mol Moles NaOH used (II) ______________mol

mol NH3(I)=mole HCl – mole NaOH = ____________mol mol NH3(II)=mole HCl – mole NaOH = ____________mol

Mass of NH3(I) _______________ g Mass of NH3(II) _______________ g

Molar Mass of NH3 ______________g/mol

%NH3(I) _______________ % %NH3(II) _______________ %

Average %NH3 = _______________ %

(a) Calculate the number of moles of standardized HCl (use exact concentration) that you STARTED with in your 250 mL

Erlenmyer Flask (EF). (b) Write the balanced equation for the reaction between HCl(aq) and NH3(g). This reaction will use up some of your HCl. (c) Write the balanced equation for the reaction between HCl(aq) and NaOH(aq). This is how you will determine how much

HCl was left over in each determination. (d) Calculate the number of moles of standardized NaOH (use exact concentration) you added to neutralize the left-over HCl

in your beaker. (e) Calculate how many moles of HCl reacted with the ammonia trapped during each thermal decomposition

(HClstart – HClleft over) (f) Calculate how many moles of ammonia were liberated from your complex. {See (b) above} (g) Determine the molecular mass of ammonia. (h) Calculate {use results from step (f)} the number of grams of ammonia liberated from your complex. (i) Using the mass of the sample of the complex you started with (from data P4 for each determination), calculate the %

ammonia in the complex (g NH3/g complex) x 100.) Calculate average % NH3 and enter in results above. Show calculations for one determination below!

SACC page 12 9. Determination of the formula of Cox(NH3)yClz

Percentages % Co = _______________ %NH3= _______________ %Cl= _______________

Whole number ratios x = ___________ y = __________________ z = __________________

Formula of Compound: __________________________

Show both Empirical formula and percent yield calculations below!

10. Problems

(a) Estimate the maximum wavelength of visible light absorbed by the purple Co-complex ion that you synthesized or used for analysis. ____________ (b) What is the oxidation state of Co in the complex ion? ________ (c) Determine the crystal field splitting energy, ∆, in units of J/photon and kJ/mol (photons). (d) Speculate on whether the complex ion is paramagnetic or diamagnetic. Draw a Crystal Field Splitting diagram and briefly, justify your choice.

SACC page 13

Advanced Study Assignments Name: ________________ Lab MW/TTH/M-TH (circle)

Day 1-Part 1. Synthesis

(a) What is the purpose of the H2O2 in the synthetic step of this experiment?

(b) Effervescence was likely observed when the H2O2 was added, suggesting the evolution of a gas. Suggest a likely

composition for the gas.

(c) Calculate the molar mass of the limiting reactant in the synthesis of Cox(NH3)yClz

SACC page 14

Advanced Study Assignments Name: ________________ Lab MW/TTH/M-TH (circle)

Day 2-Part 2. Chloride Analysis A student, Legna, massed out 10.12 g of chromium(III)chloride hexahydrate to synthesize a transition metal coordination complex similar to what you are carrying out in your laboratory synthetic effort. She obtained 7.20 g of a complex containing Chromium, Cr, ammonia, NH3, and chlorine, Cl. Samples of this compound were used to perform all analyses for the remainder of the advanced study assignments. The following data were obtained. {To the student: Please note that the synthesis and analysis of the compound in this and the following advanced study assignments are similar to but not identical to your experimental synthesis and analysis project!} Analysis 1. No heat applied Analysis2. Heat applied Mass of compound used 0.4988 g Mass of compound used 0.4632 g Mass of crucible plus AgCl 19.2859 g Mass of crucible plus AgCl 21.0926 g Mass of crucible 18.4628 g Mass of crucible 20.3286 g Mmass AgCl = 143.32 g/mol Mmass Cl- = 35.45 g/mol

(a) Determine the %Cl in each sample of the compound. Show calculations below! Not heated ______________%, Heated ______________%

(b) Calculate the ration of %Cl(heated analysis)/%Cl(unheated analysis). ______________ (c) Which Cl-analysis gives the total chloride in the complex (heated or unheated analysis)? Briefly explain.

(d) What does the ratio of %Cl(heated)/%Cl(unheated) reveal about the number and location of the chlorides in this particular compound?

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Advanced Study Assignments Name: ________________ Lab MW/TTH/M-TH (circle)

Day 3-Part 3. Spectrophotometric determination of %Cr in Crx(NH3)yClz

(a) In a similar fashion as you will conduct for Co2+ in the laboratory, the data below were recorded for a solution that is 0.50 M in Cr3+ ions using a Spectronic 20 or 20+ spectrophotometer.

%Transmittance Wavelength (nm) Absorbance 65.1 700 52.2 675 24 650 8.2 625 4.3 600 3.1 575 7.8 550 16.3 525 32.1 500 35.3 475 37.2 450 Convert % Transmittance (T) to Absorbance (A) using the relationship A = 2 – log(%T). Use Excel, or similar program, plot absorbance versus wavelength and determine the wavelength of maximum absorbance of Cr3+ ion. Attach your plot to your report. Ans. _________

(b) To determine the %Cr in the chromium-complex Legna synthesized, she massed out a 1.0240 g sample of the Cr-complex. She dissolved the sample in a volumetric flask by the procedure you will use in part 3 of the experiment and made 100.0 mL of solution. From the standard curve of Absorbance versus concentration Legna obtain a [Cr3+] = 0.03920 M. Calculate the %Cr in the complex.

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Advanced Study Assignments Name: ________________ Lab MW/TTH/M-TH (circle)

Day 4-Part 4. Volumetric determination of %NH3 in Crx(NH3)yClz

(a) A 0.2536 g sample of the Cr-complex was thermally decomposed and the NH3 released was collected in a 250 mL Erlenmyer flask containing 30.00 mL of 0.3065 M HCl solution and about 20 mL of deionized water. The solution was then titrated with standardized NaOH solution having a concentration of 0.2816 M. It required 11.92 mL of NaOH to reach the bromcresol green end point. Determine the %NH3 in the sample.

(b) Determination of the formula of Crx(NH3)yClz (i) Use the percentages obtained for Cl, Cr, and NH3 and determine the formula of the Cr-complex Legna synthesized. (ii) Calculate the percent yield of Legna’s synthetic endeavor.