Question

Which of the following solutions will alter the pH of the solution on dilution?

• A. $0.1MN{H}_{4}OH+0.1MN{H}_{4}Cl$
• B. $0.5M{H}_{2}C{O}_3+0.5MNaHC{O}_3$
• C. $1MC{H}_{3}COON{H}_4$
• D. $0.1MC{H}_{3}COONa$

Answer 1

The correct option is D $0.1MC{H}_{3}COONa$

In option(a), $N{H}_{4}OH$ is a weak base, $N{H}_{4}Cl$ is a salt of its conjugate acid. Thus, this is a basic buffer.

In option(b), $H_{2}C{O}_3$ is a weak acid and $NaHC{O}_3$ is a salt of its conjugate base. This is an acidic buffer.

In option (c), the salt is formed by a weak acid and a weak base. So, it acts like a buffer too.

In option(d),
$CH3COONa$ is the salt of a strong base sodium hydroxide NaOH and a weak acid acetic acid $C{H}_{3}COOH$.
All buffers are not affected by dilution but in the case of the salt, pH would change.

You can also reason this out by looking at the underlying equations, concentration plays a role only in the pH equation of a salt, whereas in buffer solutions, the ratio of concentrations of salt and base/acid are involved, eliminating the volume factor.