Determination of the Solubility Product
Of an Ionic Compound
The solubility product constant, Ksp, is a particular type of equilibrium constant. The
equilibrium is formed when an ionic solid dissolves in water to form a saturated solution. The
equilibrium exists between the aqueous ions and the undissolved solid. A saturated solution
contains a maximum concentration of ions of the substance that can dissolve at the solution’s
temperature. The equilibrium equation showing the ionic solid lead chloride dissolving in water is
PbCl2(s) ↔ Pb2+(aq) + 2Cl-(aq)
The solubility product expression is:
Ksp = [Pb2+][Cl-1]2
where the square brackets refer to molar concentration of the ions. A knowledge of the Ksp
of a salt is useful, since it allows us to determine the concentration of ions of the compound in a
saturated solution. This allows us to control a solution so that precipitation of a compound will not
occur, or to find the concentration needed to cause a precipitate to form.
The solubility product constant which will be determined by this experiment is that of the
strong base, calcium hydroxide, Ca(OH)2. This will be determined three different ways and the
results of each method compared.
Prelab:
1. Write a balanced net ionic equation for the dissolving of solid barium phosphate in water.
2. Write the solubility product expression of barium phosphate dissolved in water.
3. A saturated 1.0 L solution of barium phosphate has a molarity of 2.5 x 10-5 M.
a) What are the concentrations of barium ions and phosphate ions in this solution?
b) What is the concentration of barium phosphate in g/L?
c) If a100.0 mL sample of the 1.0 L barium phosphate solution is removed and the water is
then allowed to evaporate to dryness, how many grams of barium phosphate would be
collected from the sample? Show your work or explain your reasoning.
4. When 2.0 mL of 0.50 M sodium chloride solution is added to 2.0 mL of water, what is the
new sodium chloride solution concentration?
5. 2.0 mL of the diluted solution from question #3 is then added to 2.0 mL of water, what is this
new solution’s concentration? How does this compare to the original sodium chloride
solution concentration?
6. A solution of potassium hydroxide is found to have a pH of 9.45.
a) What is the concentration of hydroxide ions in this solution?
b) How does the concentration of potassium ions compare to that of the hydroxide ions?
Explain.
Procedures:
Part 1: Reaction between calcium nitrate and potassium hydroxide.
A. Prepare a series of diluted calcium ion solutions.
Arrange two, 24 well microplates so that you have 12 wells across from left to right. Put 5
drops of 0.10 M calcium nitrate in well #1. Place 5 drops of water in each of the wells #2
through #12. Next add 5 drops of 0.10 M calcium nitrate to well #2. Use an empty Beral
pipet to mix the solution thoroughly by drawing the solution into the pipet and then
squirting it back several times.
Use your empty pipet to remove the solution from well #2 and put 5 drops of this solution
into well #3. Put the remaining solution back in well #2. Mix the solution in well #3 as
before. Continue this serial dilution procedure, adding 5 drops of the previous solution to
the 5 drops of water in each well down the row until you fill the last one, #12. Mix the
solution in well #12, and discard 5 drops. Determine the concentration of solution in each
well.
B. Combine with potassium hydroxide.
Place 5 drops of 0.10 M potassium hydroxide, KOH, in the wells #1 through #12. Use
separate toothpicks to mix the combined solutions. What is the concentration of the ions
after the mixing has occurred compared to what they were like prior to the mixing?
C. Observe the precipitates and calculate Ksp.
Allow three to four minutes for the precipitates to form. Observe the pattern of
precipitation. At one point the concentration of both ions becomes too low to have any
precipitate form. The first point where this occurs represents a saturated solution.
Calculate the concentration of Ca2+ ions and OH1- ions in this well. Using these
concentrations determine the solubility product constant, the Ksp of calcium hydroxide.
Part 2: Evaporation of a saturated solution of calcium hydroxide.
1. Measure the mass of a clean, dry 100 mL beaker.
2. Transfer 5.00 mL of a saturated solution of calcium hydroxide solution to the 100 mL beaker.
3. Let the water of the solution evaporate overnight.
4. Measure the mass of the beaker and the evaporated solution.
5. Calculate the molar concentration of the calcium hydroxide solution.
6. Calculate the Ksp of calcium hydroxide.
Part 3: pH of a saturated calcium hydroxide solution.
1. Determine the pH of a 25.0 mL sample of a saturated solution of calcium hydroxide.
2. Calculate the concentration of hydroxide ions in this solution.
3. Calculate the concentration of calcium ions in the solution.
4. Calculate the ksp of calcium hydroxide.