Empirical Formula of a Lead Compound

References:

Chemistry, 4^th ed, by J. McMurry and R. Fay. Prentice Hall, 2004, Sections 2.10, 3.8, 3.9, 3.11, 3.12

Purpose:

To determine the empirical formula of a lead iodide.

Method:

In this experiment you will determine the empirical formula of a compound composed of lead and iodine. A weighed quantity of lead is dissolved with nitric acid, HNO₃(aq), solution. The resulting lead nitrate solution is then allowed to react with sodium iodide, NaI, solution to form insoluble lead iodide, This precipitate is collected by filtration, dried, and weighed. From the experimental data you can calculate the percentage composition and the ratio of moles of iodine to moles of lead in the compound, and thus determine the empirical formula.

Procedure

This experiment will be run in duplicate. The two trials should be performed simultaneously in order to save time. Record all experimental data in your notebook in clearly labeled, neat tables. Use Table 1 as a guide, but do not copy and paste it directly into your notebook.  Be sure the significant figures entered into your table match the measuring devices.

Label two clean 250 or 400 mL beakers as #1 and #2. Place beaker #1 on the balance and weigh a 0.130-0.150 g sample of granulated lead, Pb, into the beaker. This weighing must be to ±0.001 g.  Record the weight of the lead sample in your notebook. Repeat the process for beaker #2. If any lead should spill in the process of weighing, clean it up and place in the waste lead container in the lab.

In the hood where you will use it, pour out approximately 20 mL of 3 M nitric acid solution in your graduated cylinder.  If any acid runs down the outside of the bottle, rinse it off with a little water.  CAUTION: HNO₃ of this concentration can cause burns on skin or holes in clothing. Wear gloves!.  If any spills occur, rinse the affected area thoroughly with water. (When nitric acid solution touches skin, the skin can turn a deep yellow color). [Dispose of excess nitric acid in the Aqueous Waste container].

Pour half of the 3 M HNO₃ onto each sample of lead in beakers 1 and 2. Cover beaker 1 with a clean watch glass, and heat it gently over a hot plate in a fume hood until steam exits from under the watch glass. CAUTION: do not boil! Control the heating so that gas evolution continues, but do not boil. After all the lead has dissolved, add approximately 20. mL of deionized water to the beaker.  Heat the solution again until it steams, but do not boil.  Remove the beaker from the hot plate, and let it stand for several minutes.  Weigh (use the top loading balances in the lab!) approximately 0.8 g of sodium iodide onto a weighing paper.  Do not take the sodium iodide bottle into the Balance Room.  Transfer the weighed sodium iodide to a 100-mL or larger beaker, and add 40. mL of deionized water to this beaker.

Heat this solution until it steams, allow the solution to cool slightly, and then add half of it slowly, with stirring, to beaker #1. (Save the other half for beaker #2). Allow the mixture in beaker #1 to cool to room temperature while you repeat the procedure with beaker 2. At this point some of the lead iodide precipitate may consist of very fine crystals which will either clog the filter paper or pass through it. In order to cause the crystals to grow in size, heat the precipitate gently with constant stirring for 5-10 minutes, but do not boil the solution.  Then cool the mixture by placing the beaker into an ice bath.  The reaction is complete when no more precipitate forms.

Weigh a piece of filter paper to the nearest 0.001 gram and place it in a filtering funnel. Use this filter paper to filter the lead iodide precipitated in trial 1.  Use additional deionized water and a rubber scrapper to wash the precipitate and help transfer it completely from the beaker to the filter paper. Remove the filter paper from the funnel. Place the filter on a labeled watch glass and allow it to air dry, in your drawer, until next week. (Pour the liquid waste down the drain).  Repeat the filtration for the solid of Trial 2. 

When the filter paper and solid for each trial are completely dry, weigh each filter paper with solid and record the masses.

Results:

Tables 1 and 2 below are meant to to serve as guides to assist you in your data collection and calculations. They are not to be pasted into your notebook.

Perform the calculations in Table 2. 

Table 1.

Mass Data for Precipitation of Lead Iodide

Table 2.

Calculations for the Empirical Formula of Lead Iodide

Calculate the percent composition of lead and of iodide in your final product.                  

Be sure to show one complete (very neat) sample for every type of calculation that you do. If you do your work on a spreadsheet,  include a copy of the table that has your formulas.  Pay close attention to significant figures and units in all of your calculations.

Conclusions

Your conclusion must have all of the following.  Be sure to have complete sentences in an organized format.

Discussion

edited on 1/10/05 by j byrd