SCH4U
Lab: Ksp of Calcium Hydroxide
Purpose
To experimentally determine the Ksp of calcium hydroxide.
Background
Slaked lime (calcium hydroxide) has many uses in a wide variety of industries, such as dentristry,
environment, cosmetic and food. It can be made by combining lime (calcium oxide) and water. The
solubility of the lime is quite low, allowing for a saturated solution to be achieved, which can be shown as
equation 1.
Equation 1:
The concentration of hydroxide ions in solution react with potassium hydrogen phthalate (KHP,
KC8H4O4H) through a neutralization reaction, shown as equation 2.
Equation 2:
2KC8H4O4H (s) + Ca(OH)2
(aq)
 Ca(C8H4O4)2 (aq) + 2H2O (ℓ)
Materials
- calcium hydroxide solution
- burette
- pipette bulb
- phenolphthalein
- KHP(s) (molar mass KHP = 204.23 g/mol)
- 125mL Beaker
- 125 mL Erlenmeyer flask - 25.00 mL pipette
Procedure
1.
Clean a 250-mL flask, and add approximately 0.05g of KHP. Add approximately 50 – 75mL of
deionized water. Add 2 – 3 drops of phenolphthalein to the flask.
2.
Clean and prepare a burette (refer to textbook p. 728 – 729). Fill the burette with the
Ca(OH)2(aq).
3.
Titrate to the first permanent appearance of pink. Near the endpoint, add the Ca(OH)2 dropwise
to determine the total volume most accurately.
4.
Repeat steps 1 and 2 until you have three trials for which you have achieved a satisfactory end
point.
5.
Dispose of all solutions as instructed by your teacher, clean up the apparatus and complete your
calculations below.
Observations
Data
Trial 1
Trial 2
Trial 3
Trial 4
initial buret reading
final buret reading
mass of acid used
average volume of
Ca(OH)2 used
Calculations
1.
Calculate the number of moles of KHP (s) that reacted during an average titration.
2.
Determine the number of moles of OH that reacted during an average titration.
3.
Calculate the molar (i.e. mol/L) concentration of the OH-(aq) ion.
4.
Determine the equilibrium equation concentration of the Ca2+(aq) ion.
5.
Calculate the Ksp of calcium hydroxide.
6.
One published value for the Ksp of Ca(OH)2 is 5.02 x 10-6 at 25oC (1). Using this as the accepted
value, calculate the percent error of your result.
7.
Provide an error analysis for this experiment. Be sure to include the effect on your calculations.
Conclusion
Write a brief conclusion to summarize your findings.
Question
Suppose that 250 mL of your filtered Ca(OH)2 solution was mixed with 750 mL of a 1 μmol/L solution of
FeCl3 (aq). Would a precipitate form? Identify the possible precipitate and support your answer with
full calculations of ion concentrations and the Trial Ion Product. Solubility product constants can be
found on p. 746 of McGraw-Hill Ryerson Chemistry 12.