Question

Would a 1 M solution of just acetic acid resist increases to pH, decreases to pH, neither, or both? Briefly explain.

Answer 1

Answer.

A mixture of acetic acid is acidic because the Ka of acetic acid is greater than the Kb of its conjugate base acetate. It is a buffer because it contains both the weak acid and its salt. Hence, it acts to keep the hydronium ion concentration (and the pH) almost constant by the addition of either a small amount of a strong acid or a strong base. If we add a base such as sodium hydroxide, the hydroxide ions react with the few hydronium ions present. Then more of the acetic acid reacts with water, restoring the hydronium ion concentration almost to its original value:

CH3CO2H(aq)+H2O(l)⟶H3O+(aq)+CH3CO−2(aq)CH3CO2H(aq)+H2O(l)⟶H3O+(aq)+CH3CO2−(aq)

The pH changes very little. If we add an acid such as hydrochloric acid, most of the hydronium ions from the hydrochloric acid combine with acetate ions, forming acetic acid molecules:

H3O+(aq)+CH3CO−2(aq)⟶CH3CO2H(aq)+H2O(l)H3O+(aq)+CH3CO2−(aq)⟶CH3CO2H(aq)+H2O(l)

Thus, there is very little increase in the concentration of the hydronium ion, and the pH remains practically unchanged

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