Lab 20: Determination of Ksp
 This lab does NOT require a lab report.
 You will work with a partner for this lab.
I Introduction
Solubility rules are useful for predicting if a compound is soluble or not, but they are
often peppered with such ambiguous phrases as “slightly”, “very slightly”, and
“marginally”. For a more rigorous, mathematical treatment of solubility, we turn to
the solubility product, Ksp, a unique constant for each compound that tells just how
soluble the compound is. Although we generally consider calcium hydroxide, Ca(OH)2,
to be insoluble by using our solubility rules, it is actually slightly soluble in water and
dissolves as shown in the equation below:
Ca(OH)2(s) ↔ Ca2+ (aq) + 2OH– (aq)
giving an equilibrium expression of:
Ksp = [Ca2+][OH–]2
Your primary objective in this experiment is to test a saturated solution of calcium
hydroxide and use your observations and measurements to calculate the Ksp of the
compound. You will do this by titrating the prepared Ca(OH)2 solution with a standard
hydrochloric acid solution. By determining the molar concentration of dissolved
hydroxide ions in the saturated Ca(OH)2 solution, you will have the necessary
information to calculate the Ksp.
Equipment and Reagents
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Ring stand and buret clamp
Buret
Magnetic stirrer and stir bar
100-mL graduated cylinder
2 100-mL beakers
Erlenmeyer flask
Funnel, ring, and filter paper
25-mL volumetric pipet
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Saturated Ca(OH)2(aq), calcium hydroxide
0.050 M HCl(aq), hydrochloric acid
Phenophthalein indicator solution
! Warnings!
 Although relatively dilute, the hydrochloric acid and calcium hydroxide solutions
are corrosive to eyes and body tissue. Wear your goggles; aprons are
recommended. Clean up any spills with wet paper towels.
Procedure
1. Set up your lab notebook and don your safety apparel.
2. Obtain about 70 mL of a saturated calcium hydroxide solution and filter it to
remove any solid, undissolved Ca(OH)2.
3. Using a volumetric pipet, measure out exactly 25.0 mL of the filtered solution into
a 250 mL Ernlenmeyer flask. Add a magnetic stir bar and a few drops of
phenolphthalein indicator solution and place your flask on a magnetic stirrer.
4. Rinse and fill your buret with 0.0500 M HCl solution.
5. Titrate the calcium hydroxide solution until the endpoint is reached. For best
results, be sure to slow down addition of HCl to single drops when the color
changes become longer lasting.
6. Record the volume of 0.050 M HCl used and dispose of the reaction mixture down
the drain and flush with water.
7. Repeat with a second titration.
Data and Calculations (to be recorded on your lab Data Sheet)
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Procedure and observations
Volumes of HCl required to reach equivalence point (at least 2 trials)
Equation and Ksp expression for dissolving of Ca(OH)2
Net ionic equation of reaction between acid and dissolved Ca(OH) 2
[OH-], [Ca2+], and Ksp, with supporting calculations
? Questions to Answer
1. Find the accepted value of the Ksp for calcium hydroxide and compare it with your
value. Discuss the discrepancy and suggest possible sources of experimental error.
2. Why is it important to filter the solution and remove any solid calcium hydroxide
before titrating it?
3. Lead hydroxide, Pb(OH)2, has a Ksp value of 1.4 × 10-20. If you were to titrate an
identical volume (25.0 mL) of saturated Pb(OH)2 with 0.050 M HCl, would you
expect to use a larger, smaller, or same volume of HCl as you did in the titration
of Ca(OH)2? Explain your reasoning.