Alexis Musso

CHEM 111H – 202

October 2 and 9, 2012

Experiment 18

Preparation and Analysis of Lead (II) Iodide:

Recovery, Recycling, and Reuse of the Metal

Abstract

This experiment’s objective was to study the chemistry of a metal by preparing a compound and determining its formula by gravimetric analysis. In part one, PbI2 was prepared through a series of reactions starting with (PbCO3)2·Pb(OH)2 and using acids and potassium iodide. The technique used was recrystallization in which a solid was dissolved using a hot solvent, and then the solution was cooled to effect precipitation. After filtering a volume of the solution, the collected solids of PbI2 were dried then weighed to calculate percent yield. Then, in part two, purity of the PbI2 produced was determined by comparing the percent lead obtained through gravimetric analysis with the theoretical percentage. Finally, the empirical formula of PbI2 was determined. In total, the experiment was a success if the empirical formula determined for lead iodide corresponded to the theoretical empirical formula of PbI2.

Experimental

Stoichiometric Equations:

· (PbCO3)2·Pb(OH)2 + 6HNO3 → 3Pb2+ + 6NO3- + 4H2O + 2CO2 + excess H+

· Pb(NO3)2 + 2OH- → Pb(OH)2↓ + 2NO3-

· NH3 + H2O → NH4- + OH-

· Pb(OH)2 + 2HNO3 → Pb(NO3)2 + 2H2O

· Pb(NO3)2 + 2KI → PbI2 + 2KNO3

· PbI2(s) + HNO3(aq) → Pb2+(aq) + 2NO3-(aq) + H+(aq) + I-(aq)

· Pb2+ + H2SO4 → PbSO4 + H+

Mathematical Formulas:

· Theoretical yield PbI2 = g (PbCO3)2·Pb(OH)2 (

· Percent yield PbI2 =

· Weight of Pb in PbSO4 (g) = g PbSO4 (

· Percent Composition = x 100

· Theoretical % lead in PbI2 = x 100

· Percent error = x 100

· Moles of element in PbI2 =

· Mole Ratio = >> use to find empirical formula PbxIy

Reagents:

Common Name

Formula

Molar Mass (g/mole)

Density (g/cm3)

Melting Point (°C)

Boiling Point (°C)

lead carbonate

(PbCO3)2·Pb(OH)2

775.62

-

>400

-

nitric acid

HNO3

63.00

2-3 (vapor)

-42

122

ammonia solution

NH3

58.179

0.59

-77

-

sulfuric acid

H2SO4

98.08

3.4

-64

337

potassium iodide

KI

166.0

3.1

680

1330

Water

H2O

18

1.00

0

100

Source: http://www.chem.tamu.edu/class/majors/chem101h-lab/index.html, http://ull.chemistry.uakron.edu/erd/

Materials:

Part One:

· 1 small X stir bar

· 1 large X stir bar

· filter paper

· 1 Buchner funnel

· 1 suction flask

· 1 watch glass

· 1 Pasteur pipet + bulb

· 1 magnetic stirring hot plate

· 1 sand bath

· 1 support stand

· 2 magnetic stir retrievers

· 1 600 mL beaker

· 1 50 mL beaker

· 1 250 mL beaker

· 1 400 mL beaker

· Analytical balance

Part Two:

· 1 small X stir bar

· filter paper

· waste beaker

· 1 tweezers

· 1 crucible + cover

· 1 triangle

· 1 Buchner funnel

· 1 suction flask

· 1 25 mL volumetric pipet

· 1 pipet pump

· 1 Pasteur pipet + bulb

· 1 ring stand

· 1 magnetic stirring hot plate

· 1 desiccator

· 1 Bunsen burner

· 1 100 mL beaker

· 1 100 mL volumetric flask

· 1 150 mL beaker

· Analytical balance

Experimental Apparatus:

Support Stand

400 mL beaker

Magnetic Stirring Plate

Buchner funnel

Ring stand

Triangle Holder

Bunsen Burner

Crucible

Procedure

Part 1:

1. 200 mg of the lead compound was weighed into a 50 mL beaker

2. 5 mL 6 M HNO3 was added and stirred to dissolve.

3. 6 M NH3 was added in drops until a precipitate persisted.

4. The precipitate was dissolved by slowing adding 1 M HNO3 by drops.

5. Next, the solution was transferred by rinsing to a 400 mL beaker with a large stir bar.

6. 600 mg KI was weighed into a 250 mL beaker and dissolved with 50 mL H2O. Then, this mixture was added to the 400 mL beaker.

7. The total volume was brought to 150 mL and heated until the precipitate dissolved. Then, the beaker was transferred to a sand bath and the stir bar was removed.

8. After the crystal formation slowed, the beaker was placed in a larger 600 mL ice bath.

9. While the wash bottle was in the ice bath, the crystals were filtered.

10. Then, the last of the crystals were washed with the cold water onto the filter paper. The air was sucked through until the filter paper was somewhat dry.

11. The filter paper with crystals was transferred to a watch glass and stored.

12. The waste was cleaned up with all the waste going into the waste jug.

Part 2:

1. The crucible was heated to constant weight.

2. In the meantime, the watch glass, filter paper, and PbI2 were weighed.

3. Then, 200 mg PbI2 was transferred to a 100 mL beaker with stir bar. Extra PbI2 was put in the waste jar.

4. Next, the watch glass and filter paper were weighed. After, the filter paper was put in the plastic beaker on the desk.

5. 10 mL of 50% HNO3 was added to the beaker of PbI2.

6. The mixture was heated under the hood until PbI2 was dissolved and no more red-brown gas was being emitted.

7. Next, it was transferred to a 100 mL volumetric flask and diluted to the mark and mixed by inversion at least twenty times.

8. 25 mL of the solution was pipetted twice into a 150 mL beaker.

9. The beaker was cooled thoroughly and 10 mL of concentration H2SO4 was added.

10. The beaker was then warmed until colorless or white fumes formed.

11. Next, the beaker was cooled in an ice bath, filtered, and washed with a 0.5 M H2SO4.

12. The filter paper and PbSO4 were placed in the crucible and heated until the filter paper burned off.

13. The steps of heating, cooling, and weighing were repeated until constant weight.

14. The waste was cleaned up with all the waste going into the waste jug.

Results

Reduced Data Tables:

Chart 1: Preparation of Lead (II) Iodide

Weight of (PbCO3)2·Pb(OH)2 (g)

0.2230

Weight of KI (g)

0.6036

Weight of watch glass, filter paper, and PbI2 (g)

49.4345

Used weight of PbI2 product

0.2072

Weight of watch glass and filter paper (g)

49.0716

Total weight of PbI2 product (g)

0.3629

Theoretical yield of PbI2 product (g)

0.3976

Percentage yield

91.27%

Chart 2: Gravimetric Analysis

Trial 1

Trial 2

Used sample weight of PbI2 (g)

0.2072

Weight of empty, heated crucible (g)

11.5290

11.5286

Weight of crucible and PbSO4 (g)

11.5711

11.5712

Weight of PbSO4 (g)

0.0426

Weight of lead in PbSO4 (g)

0.0291

% lead in sample of PbI2

28.09%

Theoretical % lead in PbI2

44.95%

Percent error

37.51%

Empirical Formula

PbI3

Note: In determining the empirical formula, the actual was PbI2.56 which rounds up to PbI3 verse the theoretical PbI2.

Example Calculations:

· Total weight of PbI2 = (watch glass + filter paper + PbI2) – ( watch glass + filter paper)

· Known values: mass of watch glass + filter paper + PbI2 = 49.4345 g, mass of watch glass + filter paper = 49.0716 g

· Unknown variable: total weight of PbI2 product

· Total weight of PbI2 product = (49.4345 g) – (49.0716 g) = 0.3629 g

· Theoretical yield PbI2 = g (PbCO3)2·Pb(OH)2 (

· Known values: g PbCO3 = 0.2230 g, molar mass (PbCO3)2·Pb(OH)2 = 775.62 g/mol, mole ratio of Pb(NO3)2 to (PbCO3)2·Pb(OH)2 = 3:1, mole ratio of PbI2 to Pb(NO3)2 = 1:1, molar mass PbI2 = 461.00 g/mol

· Unknown variable: theoretical yield of PbI2

· 0.2230 g (PbCO3)2·Pb(OH)2 (= 0.3976 g PbI2

· Percent yield PbI2 =

· Known values: total yield of PbI2 = 0.3629 g, theoretical yield of PbI2 = 0.3976 g

· Unknown variable: percent yield

· Percent yield PbI2 = = 91.27% PbI2

· grams PbSO4 = (Weight of crucible + PbSO4) – (Weight of empty, heated crucible)

· Known values: Weight of crucible and PbSO4 = 11.5712 g, Weight of empty, heated crucible (g) = 11.5286 g

· Unknown variable: weight of PbSO4

· Weight of PbSO4 (g) = 11.5712 g – 11.5286 g = 0.0426 g PbSO4

· note: the second trials were used

· Weight of Pb in PbSO4 (g) = g PbSO4 (

· Known values: g PbSO4 = 0.0426 g, molar mass PbSO4 = 303.26 g/mol, mole ratio of Pb to PbSO4 = 1:1, molar mass Pb = 207.2 g/mol

· Unknown variable: Weight of Pb in PbSO4

· Weight Pb = 0.0426 g PbSO4 ( = 0.0291 g

· Percent Composition = x 100

· Known values: mass Pb = 0.0582 g, mass PbI2 = 0.2072 g

· Unknown variable: percent composition

· % Pb = x 100 = 28.09 % Pb

· Theoretical % lead in PbI2 = x 100

· Known values: molar mass Pb = 207.2 g/mol, molar mass PbI2 = 461.00 g/mol

· Unknown variable: theoretical % Pb in PbI2

· Theoretical % lead in PbI2 = x 100 = 44.95% Pb

· Percent error = x 100

· Known values: actual % = 28.09%, theoretical % = 44.95%

· Unknown variable: percent error

· Percent error = x 100 = 37.51% error

· Moles of element in PbI2 =

· Assuming 100 g of the compound, 28.09% Pb is 28.09 g Pb

· Known values: mass of element = 28.09 g Pb, molar mass PbI2 = 461.00 g/mol

· Unknown variable: moles of element

· # Moles = = 0.0609 mol Pb

· Mole ratio =

· Known values: moles of Pb = 0.0609 mol Pb, moles of I = 0.1560 mol I

· Unknown variable: mole ratio

· Mole ratio = = 2.56 ≈ 3

· Therefore, the ratio of Pb to I is 1:3, and the empirical formula is PbI3

Discussion

In part one, a series of reactions were carried out to implement recrystallization of PbI2, in which the solid was dissolved using a hot solvent and the solution cooled to effect precipitation. First, 5 mL 6 M HNO3 was added to (PbCO3)2·Pb(OH)2 to separate out the Pb2+ ions in the reaction: (PbCO3)2·Pb(OH)2 + 6HNO3 → 3Pb2+ + 6NO3- + 4H2O + 2CO2 + excess H+

Then, 6 M NH3 was added in drops until a precipitate persisted to transform Pb(NO3)2 to Pb(OH)2. Then, to get Pb(OH)2 back in solution, 1 M HNO3 was added by drops to slowly dissolve the precipitate. It was important to add just enough to re-dissolve Pb(OH)2 so that excess acid was not produced. Finally, about 6 mg of KI was weighed out and added to the solution in order to produce PbI2 in the reaction: Pb(NO3)2 + 2KI → PbI2 + 2KNO3. Because the limiting reagent was (PbCO3)2·Pb(OH)2, the grams were converted to moles using its molar mass. Then, because the mole ratio of Pb(NO3)2 to (PbCO3)2·Pb(OH)2 = 3:1 and the mole ratio of PbI2 to Pb(NO3)2 = 1:1, the number of moles of PbI2 was obtained and converted to grams using its molar mass.

x grams (PbCO3)2·Pb(OH)2 (

The solids collected from filtering were dried and weighed to calculate the yield of PbI2. The percent yield was then determined by comparing the actual yield to the theoretical yield of PbI2.

Next, in part two, about 200 mg of the lead (II) iodide from part one was measured out and 10 mL of nitric acid was added. The mixture was heated, transferred to a 100 mL flask, and diluted to the mark. Then, 50 mL of the solution was transferred to another beaker, 10 mL of concentrated H2SO4 was added, and the beaker was warmed until white fumes formed. It was important to note that only 50 mL of the 100 mL of solution was used in the rest of the experiment because in the data manipulation the grams of lead calculated would need to be multiplied by two to get the entire amount. After the beaker was cooled, filtered, and washed, the filter paper and PbSO4 were placed in the crucible and heated to burn off the filter paper. Once the crucible was cooled, it could be measured. This allowed the amount of PbSO4 to be calculated by difference of crucible plus PbSO4 and the empty crucible weighed at the beginning. The grams of PbSO4 determined could be converted to moles using molar mass, and since the mole ratio of Pb to PbSO4 is 1:1, this was the same number of moles of Pb, which was finally converted to grams. The grams of lead found was then multiplied by two to get the total lead in the experiment. This amount was further used to calculate percent lead in lead (II) iodide using percent composition:

Percent Composition = x 100

Finally, the purity of the lead (II) iodide produced was determined by comparing the percent lead obtained through gravimetric analysis with the theoretical percentage. Lastly, the empirical formula of lead (II) iodide was determined to be PbI3 due to a percent error of 37.51%.

The experiment was unsuccessful due to some errors. In part one, one error that may have occurred is that some of the solid PbI2 could have been lost while filtering if it got under the filter paper. In part two, an error occurred when the filter paper was disposed of in the waste jar before weighing a second time. However, the filter paper was retrieved and weighed but an inaccurate weight may have been obtained. Overall, because the solution was continually being transferred between beakers, flasks, and crucibles, continual loss of the product may have occurred adding up to a significant loss if it stuck to the pipettes and beakers from which it was being transferred or filtered each time.

Conclusion

The percent lead determined in the sample of lead (II) iodide was 28.09% while the theoretical percent was 44.95%, which gives a percent error of 37.51%. The empirical formula determined for lead (II) iodide was PbI3. Therefore, the experiment was fairly

unsuccessful because the theoretical empirical formula is PbI2.