Lab #20 – Determination of Ksp with the Spec-20
Name
Background
The solubility product constant, Ksp, for a substance is the equilibrium constant for the dissolving of the
substance. In this lab, we will determine the Ksp for PbI2:
1 PbI2(s)  1 Pb+2(aq) + 2 I-(aq)
Ksp = [Pb+2] [I-]2
In this procedure, we will combine Pb+2 and I- ions in 5 different proportions to create an equilibrium mixture.
We will use the Spec-20 to determine the equilibrium concentration of I- which will also allow us – one way or
another – to determine the equilibrium concentration of Pb+2. We can use these to calculate the value of the Ksp.
In four of our mixtures we will analyze the supernatant solution containing the remaining Pb+2 and I- ions. In
the fifth mixture we will collect the precipitate formed and redissolve it. The resulting solution will then be
analyzed. We will therefore be checking the equilibrium from both directions; four from the right side of the
equation above, and one from the left.
We can only use the Spec-20 to determine the concentration of colored species. Since the I- ion is colorless, we
will first oxidize it into the reddish-purple I2. This will not require any additional calculating on our part, as the
Spec-20 calibration data will have been gathered by oxidizing several solutions of known [I-] using the same
procedure for processing the unknown samples.
Procedure
Use three small beakers to obtain approximately 35 mL of 0.012 M Pb(NO3)2 in 0.20 M KNO3, 30 mL of 0.030
M KI in 0.20 M KNO3, and 10 mL of 0.20 M KNO3. Label five medium test tubes #1 – 5 and use the Mohr and
aliquot pipettes as demonstrated by your instructor to dispense the following combination of reagents into the
corresponding test tube.
Test Tube #
1
2
3
4
5
mL of 0.012 M Pb(NO3)2
5.0
5.0
5.0
5.0
10
mL of 0.030 M KI
2.0
3.0
4.0
5.0
10
mL of 0.20 M KNO3
3.0
2.0
1.0
0
0
Test tubes #1-4 should be stoppered and thoroughly shaken every several minutes while you process test tube
#5. In test tube #5, combine the reagents, measuring quickly and approximately with a graduated cylinder.
Shake thoroughly for several minutes and allow the precipitate to settle. Decant ¾ of the solution into the waste
beaker and pour the remaining solution and precipitate into a small test tube. Place this tube and a balancing
tube into the centrifuge and spin for a minute or two. Pour off the liquid (waste beaker), add 3 mL of KNO3,
shake to wash the ppt, then centrifuge again. Pour off the liquid, then cover the ppt with enough 0.20 M KNO3
to fill the test tube ¾ full. Shake this mixture every minute or so for 10 minutes to redissolve some of the ppt.
When each mixture has been shaken for at least 15 minutes, let the tubes stand for several minutes to settle.
Pour the supernatant solution from each medium test tube into a small test tube, filling the small tube ¾ full.
Discard the remaining solution and ppt into the waste beaker. Centrifuge the small test tubes for 3 or 4 minutes.
Pour the new supernatant solution into another small test tube; centrifuge again if more PbI2 solid remains.
When the solution is clear, dab the surface of the solution with a KimWipe to remove any floating PbI2
particles.
To oxidize the I- ions in each tube: Pipette 3.0 ml of a solution into a cuvette. Pipette 3.0 mL of KNO2 into the
cuvette and add 2 drops of 6 M HCl. Shake thoroughly and measure the absorbance of the colored I2 solution at
525 nm. Be sure to get the calibration data for the Spec-20 you used! Repeat for the remaining four test tubes.
Data and Calculations
1. Use the calibration data and linear regression to determine the Beer’s Law constant:
slope = k =
[I-] = k · Absorbance
correlation coefficient = r =
2. Data
Test Tube #
1
2
3
4
5
mL 0.012 M Pb(NO3)2
Sat’d PbI2(aq)
mL 0.03 M KI
Sat’d PbI2(aq)
mL 0.20 M KNO3
Sat’d PbI2(aq)
total volume
Sat’d PbI2(aq)
Absorbance
equilibrium [I-]
Show a sample calculation of the equilibrium [I-] for test tube #1:
3. Calculations
Test Tube #
1
2
3
4
5
X
X
X
X
X
X
initial moles Pb+2
initial moles Iequil. moles Imoles I- in ppt
moles Pb+2 in ppt
equil. moles Pb+2
equilibrium [Pb+2]
Ksp of PbI2
For test tube #1, show the following calculations:
moles I- in ppt
moles Pb+2 in ppt
equil moles Pb+2
Ksp
Prelab summary
Procedural Tips: Use the 5 mL aliquot pipette for the Pb(NO3)2 solution. Use the Mohr pipette for the KI.
Read carefully: This procedure uses both potassium nitrate KNO3 and potassium nitrite KNO2.
Don’t forget to record the calibration data for your Spec-20.
Read transmittance and calculate absorbance. (Abs = 2 – log %T = log decimal T)
Safety: The centrifuge can only be used in the fume hood with the hood lowered. The centrifuge must be
balanced with an even number of test tubes with the same amount of liquid in each.
Disposal: All solutions and ppt’s containing lead must be disposed of in the waste beaker in the fume hood.
Excess KI and KNO3 solutions may be poured down the sink with rinsing.
Prelab Questions
1. A Spec-20 was used to determine the concentration of X in a solution.
a. Use the following data to determine the Beer’s Law constant as per [X] = k · Absorbance. Describe the
process in your own words.
[X], M
Absorbance
1.0 x 10-4
.562
2.0 x 10-4
1.05
3.0 x 10-4
1.55
2. A yellow precipitate of PbI2 forms when 5.0 mL of 0.012 M Pb(NO3)2 are mixed with 5.0 mL of 0.030 M KI.
The equilibrium [I-] is found to be 0.007 M. Use this data to determine the Ksp of PbI2. Show all work.
initial moles Pb+2
initial moles Iequil. moles Imoles I- in ppt
moles Pb+2 in ppt
equil. moles Pb+2
equilibrium [Pb+2]
Ksp of PbI2