If an excess of a sparingly soluble solid is shaken with water, an equilibrium is set up between the solid and the aqueous ions. For example with calcium hydroxide (solubility 1.53 x 10-2 mol dm-3 at 25oC) the equilibrium is
Ca(OH)2 (s) Û Ca2+(aq) + 2 OH- (aq)
The solubility product Ks is defined for this system as
Ks = [Ca2+][OH-]2
Ks is of course an equilibrium constant, constant at constant temperature. The addition of either Ca2+ or OH- ions to a saturated solution of calcium hydroxide will cause precipitation of solid calcium hydroxide as is predicted by le Chatelier’s Principle. This fact is made use of in quantitative (gravimetric) precipitation reactions, where an excess of the precipitating reagent is usually added to reduce the solubility of the precipitate as much as is possible.
Preparation of the solutions.
In a 250 cm3 conical flask or a similar-sized stoppered bottle place 2 – 3 spatula-ends of calcium hydroxide and 150 cm3 of pure water. Stopper the container and shake for about 10 minutes. Filter this solution into a clean dry flask, and stopper to prevent the absorbtion of carbon dioxide.
In a second flask or bottle place 150 cm3 of standard 0.100 mol dm-3 sodium hydroxide solution and a similar amount of calcium hydroxide, and treat as above.
Titrations.
1 Pipette 25.0 cm3 of the calcium hydroxide solution into a 250 cm3 conical flask, add 3 – 4 drops of screened methyl orange (or methyl orange) indicator, and titrate with standard 0.0100 mol dm-3 hydrochloric acid solution. Repeat to obtain three consistent titres.
2 Pipette 25.0 cm3 of the calcium hydroxide dissolved in sodium hydroxide solution inot a conical flask as titrate as above.
Results:
(a) Calcium hydroxide dissolved in water:
| Final volume/cm3 | ||||
| Initial volume/cm3 | ||||
| Titre/cm3 |
Mean titre/cm3:
(b) Calcium hydroxide dissolved in 0.100 mol dm-3 sodium hydroxide solution:
| Final volume/cm3 | ||||
| Initial volume/cm3 | ||||
| Titre/cm3 |
Mean titre/cm3:
Calculations:
For each solution, calculate
(a) the concentration of the hydroxide ions,
(b) the concentration of calcium ions,
(c) the solubility of the calcium hydroxide.