Question

How does a mixture of ${\mathrm{CH}}_3\mathrm{COOH}$ and ${\mathrm{CH}}_3\mathrm{COONa}$ act as a buffer when a small amount of $\mathrm{NaOH}$ is added?

Answer 1

Buffer solution:

• An aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa is known as a buffer solution.
• Its pH changes very little when a small amount of strong acid or base is added to it.
• A mixture of a weak acid and its salt with a strong base serves as an acidic buffer.
• $\underset{\mathrm{Acetic}\mathrm{acid}}{{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\to \underset{\mathrm{Acetate}\mathrm{ion}}{{\mathrm{CH}}_3{\mathrm{COO}}^{–}\left(\mathrm{aq}\right)}+\underset{\mathrm{Hydronium}\mathrm{ion}}{{\mathrm{H}}_3{\mathrm{O}}^{+}\left(\mathrm{aq}\right)}$
• ${\mathrm{CH}}_3\mathrm{COOH}$ is a weak acid, while ${\mathrm{CH}}_3{\mathrm{COO}}^{–}$ is a weak base.
• On adding $\mathrm{NaOH}$ to the system: $\underset{\mathrm{Acetic}\mathrm{acid}}{{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)}+\underset{\mathrm{Sodium}\mathrm{hydroxide}}{\mathrm{NaOH}\left(\mathrm{aq}\right)}\to \underset{\mathrm{Acetate}\mathrm{ion}}{{\mathrm{CH}}_3{\mathrm{COO}}^{–}\left(\mathrm{aq}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\mathrm{Sodium}\mathrm{ion}}{{\mathrm{Na}}^{+}\left(\mathrm{aq}\right)}$

Since some of the acetic acids react with the ${\mathrm{OH}}^{–}$ from the base and convert it to water, the pH will not change as drastically.