The ionisation of a weak acid such as acetic acid is suppressed by the addition of sodium acetate because
Correct answer: B. the added acetate ions shift the ionisation equilibrium back towards the un-ionised acid
- 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.
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About Common Ion Effect
The common ion effect is the reduction in the ionisation of a weak electrolyte or the solubility of a sparingly soluble salt when a solution already contains one of its ions. Questions apply Le Chatelier's principle, equilibrium constants and solubility product expressions to predict the shift and calculate concentrations.
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