A.P. CHEMISTRY
DETERMINATION OF SOLUBILITY PRODUCT OF PbI
2
INTRODUCTION:
In this experiment you will determine the solubility product of lead iodide, PbI
2
. Lead iodide is relatively insoluble, having a solubility of less than 0.002 mole per liter at 20  C. The equation for the solution reaction of PbI
2
is:
PbI
2
(s)  Pb 2+ (aq) + 2 I (aq) (1)
The solubility product expression associated with this reaction is:
Ksp = [Pb 2+ ][I ] 2 (2)
Equation 2 implies that in any system containing solid PbI
2
in equilibrium with its ions, the product of
[Pb 2+ ] times [I ] 2 will at a given temperature have a fixed magnitude, independent of how the equilibrium system was initially made up.
In the first part of the experiment, known volumes of standard solutions of Pb(NO
3
)
2
and KI will be mixed in several different proportions. The yellow precipitate of PbI
2
formed will be allowed to come to equilibrium with the solution. The value of [I ] in the solution will be measured experimentally. The [Pb 2+ ] will be calculated from the initial composition of the system, the measured value of [I ], and the stoichiometric relation between Pb 2+ and I in Equation 1.
By mixing the solutions as we have described, we approach equilibrium by precipitating PbI
2
and measuring the concentrations of I and Pb 2+ remaining in the solution. We will also carry out the reaction in the other direction, by first precipitating PbI
2
, washing it free of excess ions, and then dissolving the solid in the inert salt solution. Under such conditions the concentrations of Pb 2+ and I in the saturated solution will be related by Equation 1, since both ions come from pure PbI
2
. From the measured value of [I ] in the saturated solution, we can calculate [Pb 2+ ] immediately.
The concentration of I ion will be found spectrophotometrically. Although the iodide ion is not colored, it is relatively easily oxidized to I
2
, which is brown in water solution. Our procedure will be to separate the solid PbI
2
from the solution and then to oxidize the I in solution with potassium nitrite, KNO
2
, under slightly acidic conditions, where the conversion to I
2
is quantitative. Although the concentration of I
2
will be rather low in the solutions you will prepare, the absorption of light by I
2
in the vicinity of 525 nm is sufficiently intense to make accurate analyses possible.
In all of the solutions prepared, potassium nitrate KNO
3
(note this distinction between KNO
2
and KNO
3
!) will be present as an inert salt. This salt serves to keep the ionic strength of the solution essentially constant at 0.2 M and promotes the formation of well-defined crystalline precipitates of PbI
2
.

EXPERIMENTAL PROCEDURE
DAY 1 – Mixing solutions and obtaining a saturated solution of PbI
2
in 5 test tubes:
1. Label five regular test tubes 1 to 5, either with labels or by noting their positions in your test tube rack.
2. Into the 5 test tubes, using a buret transfer the correct volumes of 0.0120 M Pb(N0
3
)
2
in KNO
3
, 0.0300
M KI in KNO
3
, and 0.200 M KNO
3
as summarized in the table below.
Note the total volume in each test tube must be 10.00 mL.
IN THIS EXPERIMENT IT IS ESSENTIAL THAT THE VOLUMES OF REAGENTS USED TO
MAKE UP THE MIXTURES IN TEST TUBES 1 TO 4 BE MEASURED ACCURATELY.
Volumes of Reactants Used in Precipitating PbI
2
(mL)
Test Tube 0.0120 M Pb(NO
3
)
2
(in 0.20 M KNO
3
)
0.0300 M KI
(in 0.20 M KNO
3
)
0.200 M KNO
3
1
2
3
4
5.00
5.00
5.00
5.00
10.00
2.00
3.00
4.00
5.00
10.00
3.00
2.00
1.00
0.00
0.00 5
3. Once each tube contains the correct volumes of each solution, shake them vigorously for approximately
15 minutes.
4. Let the tubes stand for three to four minutes to let the solid settle.
5. Do the additional steps below for test tube five. The rest of the test tubes can go on to part 2.
IT IS ESSENTIAL THAT ALL FIVE MIXTURES BE SHAKEN THOROUGHLY SO THAT
EQUILIBRIUM CAN BE ESTABLISHED. INSUFFICIENT SHAKING OF THE FIRST FOUR
TEST TUBES WILL RESULT IN NOT ENOUGH PbI
2
WILL DISSOLVE TO ATTAIN
EQUILIBRIUM.
TEST TUBE 5 -: ONE PARTNER SHOULD COMPLETE THESE STEPS WHILE THE SECOND
PARTNER STARTS ON THE NEXT PAGE.
1. After letting the solid settle for a few minutes, decant and discard three-fourths of the supernatant solution. (Keep a beaker at your table for waste solution. At the end of the lab period pour all your waste solution into your instructor’s waste container.)
4. Transfer the solid PbI
2
and the rest of the solution to a small test tube and centrifuge for three minutes.
5. Discard the liquid in your waste container, retaining the solid precipitate in the small test tube.
6. Add 3 mL 0.20 M KNO
3
and shake to wash the solid free of excess Pb 2+ or I .
7. Centrifuge again, and discard the liquid. By this procedure you should now have prepared a small sample of essentially pure PbI
2
in a little KNO
3
solution.
8. Add 0.20 M KNO
3
to the solid until the tube is about three-fourths full. Shake well at several one minute intervals (~ 15 minutes) to saturate the solution with PbI
2
.
4. Let the tube stand for three to four minutes to let the solid settle.
ONCE YOU COMPLETE THESE STEPS FOR TEST TUBE 5 YOU NEED TO CONTINUE ON THE
NEXT PAGE.

PART 2: Analyzing the saturated PbI
2 solution to obtain [I ].
ALL Tests Tubes (1-5):
1. Pour the supernatant liquid in each test tube into a small dry test tube until it is three-fourths full and centrifuge for about three minutes to settle the solid PbI
2
.
2. Pour the liquid into another small dry test tube; if there are any solid particles or yellow color remaining in the liquid, centrifuge again.
3. When you have a clear liquid, dip a small piece of clean, dry paper towel into the liquid to remove floating PbI
2
particles from the surface.
(Note: If you are “done” for the day, cover each of your small test tubes with parafilm or a rubber stopper.
Place carefully in your drawer. If you have a ChemX drawer place on the counter AWAY FROM the windows.)
******Do not go on, if there is little time let in the block.**************
4. Obtain a set of six cuvettes from your instructor.
5. Transfer 3.0 mL of 0.020 M KNO
2
, potassium NITRITE (NOT KNO
3
, potassium nitrate), into a clean, dry spectrophotometer tube and add 2 drops of 6 M HCl.
6. Then, using a disposable pipet, add enough of the clear centrifuged solution (about 3 mL) to fill the spectrophotometer test tube just to the level indicated on the tube.
7. Shake gently to mix the reagents and then measure the percent transmittance of the solution as directed by your instructor.
8. The calibration curve will allow you to determine directly the concentration of I- ion that was in equilibrium with PbI
2
. Be sure that the cuvette is clean and clear of bubbles prior to measuring the % transmittance.
9. Rinse the cuvettes with distilled water, then alcohol, and finally with acetone. The cuvette should dry in about two minutes. Return cuvettes to your teacher when finished.
Tests Tubes 1  4
Pb 2+ + 2 I  PbI
2
Test Tube 5
PbI
2
 Pb 2+ + 2 I -
(compute from data)
(compute with ICE) compute from spectrophotometer w/ICE chart
0 0
+x +2x x 2x from spectrophotometer

DATA
Test Tube number
Volume 0.0120 M Pb(NO
3
)
2
( ± )
Volume 0.0300 M KI ( ± )
1
_______
_______
2
_______
_______
3
_______
_______
4 5
_______ Saturated
solution
_______ of PbI
2
Volume 0.0120 M KNO
3
( ± ) _______
Total volume of solution in test tube ( ± ) _______
_______
_______
_______
_______
_______
_______ _______
% Transmittance of solution ( ± )
[I -
_______
] (mol/L) – from calibration curve (3 sig figs) _______
CALCULATIONS – complete to two decimal places
_______
_______
_______
_______
_______ _______
_______ _______
Initial moles of Pb 2+
Initial moles of I -
Number moles of I -
Number moles of I --
at equilibrium
precipitated
Number moles of Pb 2+
Number moles of Pb 2+
[Pb 2+ ] at equilibrium
precipitated
at equilibrium
_____x10
_____x10
_____x10
_____x10
_____x10
_____x10
-5 _____x10
-5 _____x10
-5 _____x10
-5 _____x10
-5 _____x10
-5 _____x10
-5 _____x10
-5 _____x10
-5 _____x10
-5 _____x10
-5 _____x10
-5 _____x10
-5
-5
-5
-5
-5
-5
_____x10
_____x10
_____x10
_____x10
_____x10
_____x10
-5
-5
-5
-5
-5
-5
_____x10 -5 _____x10 -5 _____x10 -5 _____x10 -5 _____x10 -5
Ksp of PbI
2
(3 sig figs)
Average Ksp (include unit)
% Variance
_____x10
Difference between largest
and smallest Ksp Value
Average Ksp
-5 _____x10
__________
-5 _____x10 -5 _____x10 x 100% = ____________% (2 sig figs)
-5 _____x10 -5

AP Chemistry
Pre-lab: Determination of Ksp for PbI
2
Name: _______________________
Hour: _______ Date: ________
TO BE COMPLETED AND HANDED IN PRIOR TO STARTING THE LAB.
1. State in words the meaning of the solubility product equation for PbI
2
: that is,
Ksp = [Pb 2+ ] [I ] 2 . _____________________________________________________
_____________________________________________________________________
_____________________________________________________________________
2. Name two conditions needed for this to be useful: ____________________________
_____________________________________________________________________
_____________________________________________________________________
3. When 5.0 mL of 0.012 M Pb(NO
3
)
2
are mixed with 5.0 mL of 0.030 M KI, a yellow precipitate of PbI
2 forms. Solve all parts of Question 3 to two significant figures.
________________ A. How many moles of Pb 2+ are initially present? (See section 1, suggestions regarding calculations.)
________________
________________
B. How many moles of I are originally present (Section 2)?
C. In a colorimeter the equilibrium solution is analyzed for I concentration is found to be 8.0 x 10 -3
, and its
mol/L. How many moles of I are present in the solution (10.0 mL) (Section 3)?
-
________________
________________
________________
________________
________________
D. How many moles of I units for Ksp.
precipitated (Section 4)?
E. How many moles of Pb
F. How many moles of Pb
2+
2+
precipitated (Section 5)?
are left in solution (Section 6)?
G. What is the concentration of Pb
H. Find a value for Ksp of PbI
2
2+ in the equilibrium solution (Section 7)?
from these data (Section 8). Include the

** You do not have to do this page. This is to help guide you through the calculations.
SUGGESTIONS REGARDING CALCULATIONS
PbI
2 (s)
 Pb 2+
(aq)
+ 2 I -
(aq)
Ksp = [Pb 2+ ][I ] 2
Complete calculations for Tube #1 and use as a guide for the other tubes.
In test tube 1 to 4, calculations must be based on the amounts of reagents which were used. In test tube 5,
Sections 1-6 do not apply; the solution is saturated with PbI
2
, so initial amounts of reagents are not relevant.
Several of the calculations depend upon the definition of molarity. For species A,
M
A
= no. moles A or no. moles of A = M
A
x volume (liters) volume of soln.
(6)
1.
The initial number of moles of Pb 2+ is equal to the moles in the Pb(NO
3
)
2
solution that was used.
What is [Pb
2.
What is [I -
2+ ] in that solution? _______ M. What volume was used? _____ mL; _____ Liters.
Use equation 6 to find the initial number of moles of Pb 2+ .
] in the KI solution? ______M. What volumes were used? ____ liters. Use equation 6 to find the initial numbers of moles of I present.
3.
Here we need to find the number of moles of I in each solution at equilibrium. What is the total volume of each solution? ____ mL; ______ Liters. From [I ] as measured at equilibrium, and the solution volume, find the number of moles of I in each solution at equilibrium, again using Equation
4.
6.
The number of moles of I -
No. moles I -
in solution at equilibrium is given by the equation:
in soln. = (initial no. moles I ) – (no. moles I precipitated)
From the no. of moles of I you started with (section 2) and the number remaining in solution
(section 3), you can easily obtain the number of moles of I that precipitated.
5.
The number of moles of Pb 2+ that precipitated is related to the number of moles of I that precipitated, since they both come down in the PbI
2
. What is the relationship? ________________
Calculate the number of moles Pb 2+ that precipitated from the number of moles of I that precipitated.
6.
The number of moles Pb 2+ in solution at equilibrium must equal the number initially present minus the number that precipitated. Make that calculation for tubes 1-4.
7.
What was the total volume of solution in test tube 1? _____ mL; ______Liters. Use Equation 6 to find [Pb 2+ ] in the solution in test tube. In the solution in test tube 5, the Pb 2+ and the I both come from dissolving PbI
2
, so the concentrations of those two ions are related. What is the relationship?
______________________ Calculate [Pb 2+ ] from [I ] in tube 5.
8.
From [Pb 2+ ] and [I ] at equilibrium, find Ksp for PbI
2
for each of the solutions in test tubes 1 to 5, using equation 2.