The Analysis of Lead(Ii) Iodide

Introduction

This experiment is based upon a discovery made through the isolation of PbI2 using a particular scientific method, which studies ratios of lead to iodine. Precipitation reactions occur when oppositely charged ions attract and form an insoluble ionic solid. This experiment examines the precipitation reaction between lead (II) nitrate, Pb(NO3)2, and potassium iodide, KI. Both are dissolved in water and the reaction between these solutes will produce a water insoluble yellow solid referred to as the precipitate.

Any substance with solubility less than 0.01 mol/L is considered insoluble. The formula of the insoluble product, lead iodide (PbI2), can be written by balancing charges. The second product of this reaction, potassium nitrate (KNO3), will remain in the solution. Balancing this reaction requires two I- ions for each Pb2+. Since Pb(NO3)2 contains Pb2+ ions and KI contains I- ions and we know that opposites attract, the formula for the insoluble product must be PbIn. The goal of this experiment is to determine the actual value of n in the formula for lead iodide using the same scientific method as performed at the time of its’ discovery.

Methods

The mass of each piece of laboratory equipment (test tube & rack) used in this experiment was calculated prior to the addition of any substance or solution. Using a clean sterile pipette, 2mL of 1.00 M KI solution was transferred to a clean dry test tube and weighed using an analytical balance. Followed by the addition of 2mL of 0.500 M Pb(NO3)2. The test tube was weighed again with the above-mentioned contents. The contents were then mixed, centrifuged for 2 minutes, followed by the removal and proper disposal of the supernatant using a sterile pipette.

Once the supernatant was discarded, 3mL of DI H2O were added to the test tube using a sterile pipette and centrifuged for 2 minutes. Supernatant was removed and properly discarded. Once again, 3mL of DI H2O were added to the test tube and centrifuged by 2 minutes followed by the removal of the supernatant. The test tube was then placed in a 50mL beaker, which was then placed inside of a 110(C oven for 30 minutes. The test tube with the remaining contents was allowed to cool to room temperature then weighed using an analytical balance. The test tube was then disposed of properly.

Results

The point where the values intersect on the graph for the Wednesday’s lab group is 0.553 mmoles Pb(NO3)2 at 0.3863 g of the precipitate (Fig 1). The Wednesday & Monday class data reflects 0.9 g of precipitate to 1.125 mmoles of PB(NO3)2 (Fig 2). The mole ratio is 2 mmol KI: 1 mmol Pb(NO3)2. According to the class data, the formula of the lead iodide precipitate is PbI2.

Limiting Reactant: the calculated ratio of 2 KI: 1 Pb(NO3)2 indicates that this reaction requires two molecules of potassium iodide in order for the lead nitrate to completely react. We began the experiment with a mass of 2.26 g of each compound.

(2.26 g Pb(NO3)2)(1 mol Pb(NO3)2/331.2 g Pb(NO3)2) = 6.82 x 10-3 mol Pb(NO3)2 (2.26 g KI)(1 mol KI/166.00 g KI) = 1.4 x 10-2 mol KI (2)(0.00682) = 0.01364

All of the lead nitrate will be consumed; therefore it is the limiting reactant.

Balanced Molecular Equation: Pb(NO3)2 (aq) + 2 KI (aq) ( PbI2 + KNO3 (aq)

Complete Ionic Equation: Pb2+ (aq) + 2 NO3- (aq) + 2 K+ (aq) + 2 I- (aq) ( PbI2 (s) + 2 K+ (aq) + 2 NO3- (aq)

Net Ionic Equation: Pb2+ (aq) + 2 I- (aq) ( PbI2 (s)

Theoretical yield: 0.9 g PbI2 = (0.014 mol KI)(1 mol PbI2/ 2 mol KI)(461.009 g PbI2/1 mol PbI2) = 3.23g PbI2

Percent yield: (09g PbI2/3.23g PbI2)(100) = 28%

Discussion

According to our calculations, the formula of lead iodide has been accurately measured. The calculated mole ratio is 2 KI: 1 Pb(NO3)2, which matches ratios indicated by previous experiments. The limiting reactant in this experiment is lead nitrate and the formula for the insoluble solid, lead iodide, is PbI2.

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Fig 1 Mass of the insoluble precipitate formed from the precipitation reaction of lead (II) nitrate & potassium iodide in relation to the millimoles of lead (II) nitrate according to calculations from Wednesday’s lab group.

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Fig 2 Mass of the insoluble precipitate of the precipitation reaction of lead nitrate & potassium iodide in comparison to the millimoles of lead (II) nitrate calculated by students in the Monday & Wednesday lab groups.