Solubility product constant of sparingly soluble salt can be experimental determined at room temperature by investigating its solubility. For example, the experimental determination of solubility product constant of calcium(II) hydroxide, Ca(OH)2 is given in Activity 28.3.
Pure Ca(OH)2, Two titration/conical flasks, 0.05 M Hydrochloric acid, Burette and Filkration set. Procedure
1. Record the room temperature by putting thermometer in water.
2. Weigh about 10.0 g of Ca(OH)2 solid and put it in 500 cm3 of distilled water taken in conical flask. Stir, shake and keep the contents overnight with proper cover on it or shake at least for 1 hour and filter the solution to collect filterate in clean beaker. Cover it.
3. Rinse and fill burette with 0.05 M Hydrochloric acid solution.
4. Pipette out 25 cm3 of the filterate into titration flask, add a drop of phenolphthalein indicator and titrate it against 0.05 M HCl solution till end point (pink to colourless) is reached. Repeate the titration and record the titre value (V 1 cm3)
The chemical equation is
Ca (OH)2 (aq) + 2HCl à CaCl2 (aq) + 2H2O
Let molarity of Ca(OH)2 be M2
Volume of Ca(OH)2 taken (V 2) = 25 cm3
Molarity of HCl solution M1 = 0.05
Volume of HCl used V 1 =
According to Molarity equation = M1 V1 / 2 = M2 V2 / 1
or M2 = M1 V1 / 2 = 0.05 x V1 / 25 = x (suppose)[Ca 2+(aq) = x mol -3 ; [ OH ( aq) = 2x mol dm -2
K sp = [ Ca 2+] [ OH]2 = x x (2x)2 = 4x3
Solubility of Ca (OH)2 = x mol dm -3