Free Common Ion Effect MCQs with Answers

1. The solubility of silver chloride in water decreases when sodium chloride is added. This is an example of

• A. the common ion effect
• B. the buffer action
• C. hydrolysis
• D. the catalytic effect

Explanation: Sodium chloride supplies extra chloride ions, and since the ionic product must not exceed Ksp the equilibrium shifts towards the undissolved solid, so less silver chloride can dissolve. This is Le Chatelier's principle applied to a solubility equilibrium and is used deliberately to wash a precipitate without dissolving it. Adding an ion not present in the salt would not have this effect.

2. The ionisation of a weak acid such as acetic acid is suppressed by the addition of sodium acetate because

• A. sodium acetate is a strong acid
• B. the added acetate ions shift the ionisation equilibrium back towards the un-ionised acid
• C. sodium ions react with the acid
• D. the temperature of the solution falls

Explanation: Sodium acetate is fully ionised and floods the solution with acetate, the common ion, so by Le Chatelier's principle the acid dissociates less and the hydrogen ion concentration falls. This mixture of a weak acid with its salt is exactly what makes an acidic buffer work. Sodium ions are spectators and take no part in the equilibrium.