Empirical Formula of a Lead Compound
To determine the empirical formula of a lead iodide compound by mass analysis.
This experiment will be run in duplicate. The two trials should be performed
simultaneously in order to save time. Record all experimental data directly in your
notebook in clearly labeled, neat tables. Be sure the significant figures entered into
your table reflect the measuring device used to make the measurement.
Label two clean 250 or 400 mL beakers as #1 and #2. Place beaker #1 on the balance
and, using proper reagent handling and weighing techniques, precisely weigh between
0.130-0.150 g sample of granulated lead (Pb) into the beaker. This weighing must be
at least to ±0.001 g. Record the exact mass 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 it in the waste lead container in the lab.
In the hood where you will use it, measure 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 (approx. 10
mL in each beaker). Cover beaker #1 with a clean watch glass placed with the concave
side down into the beaker, and heat the beaker 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
using tongs, and let it stand for several minutes. Weigh approximately 0.8 g of sodium
iodide (NaI) onto a weighing paper. Use the brush provided to clean up any sodium
iodide spills immediately as this substance slowly reacts with the balance pans and
ruins them. 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
(approx. 20 mL) 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 that has
formed 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
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 a small amount of
additional deionized water along with a rubber scrapper to wash the precipitate and help
transfer it completely from the beaker to the filter paper. Once the solution has run
completely through the filter, 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.
Next week, when the filter paper and solid for each trial are completely dry, weigh each
filter paper with solid and record the masses. Then you will be able to complete your
Record your raw data for each trial (mass lead, mass filter paper, mass lead iodide)
directly into your notebook. You will need to create a data table for this.
Create an Excel Spreadsheet that systematically shows the necessary calculations
used to determine the empirical formula of the product compound. Include a copy of the
spreadsheet in your notebook. See the Background Information for additional help with
the calculations for this experiment.
In your Results Section, be sure to show one complete (very neat, explicit) sample for
every type of calculation that you use. (Not just the result, but the whole mathematical
equation used to calculate the result.) Pay close attention to significant figures and
units in all of your calculations. Use meaningful labels!
In the Results Section of your notebook, write a balanced reaction for the formation of
your solid product based on YOUR results. Also, write the net ionic equation for this
reaction beneath it.
Based on YOUR RESULTS, what is the empirical formula of lead iodide? Based on the
precision of your two trials (standard deviation), comment on the uncertainty of the
formula you determined. Based on literature references, does your determined formula
have validity? Be sure to cite any references you use.
Edited on 8-14-09 by S. Phillips.