Question

When some amount of sodium acetate is further added to a mixture of acetic acid and sodium acetate, then pH of the solution:
(A)- Increases
(B)- Decreases
(C)- Remains same
(D)- None of the above can be predicted from the given info

Answer 1

Hint: The solution of acetic acid and sodium acetate forms an acidic buffer. Buffer solutions are those solutions which resist the change in pH in addition to a small amount of an acid or base. Buffer solutions resist change in pH due to common ion effects.

Complete step by step solution:
First we will define an acidic buffer.
An acidic buffer is a solution of a weak acid with a strong salt of its conjugate base. The addition of the acid to the solution does not affect its pH on account of common ion effect.
Now, in the given question sodium acetate is added to the acidic buffer. Sodium acetate is a strong salt and it will completely dissociate in the solution. Due to this the ionization of acetic acid is suppressed due to the common ion effect and the concentration of sodium increases. Hence, the solution becomes basic and the pH increases.
     \[\begin{align}
  & for\,an\,acidic\,buffer \\
 & pH=\,p{{k}_{a}}\,+\,\log \dfrac{\left[ sodium\,acetate \right]}{\left[ acetic\,acid \right]} \\
\end{align}\]
Hence, when concentration of sodium acetate is increased, pH increases.
So, the correct answer is “Option A”.

Note: Students can get confused and mark option (C) as the answer because of the presence of a buffer solution. But it should be noted that a strong salt is being added, not an acid. Hence, pH will increase. If a salt is being added then there will be complete dissociation whereas if an acid is added then incomplete dissociation will occur. This is the reason why the pH increases on adding a strong salt but not when a weak acid is added.