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Which of the following pairs constitutes a buffer :-
A
$HNO_2$ and $NaNO_2$
B
NaOH and NaCl
C
$HNO_3$ and $NH_4NO_3$
D
HCl and KCl
Detailed Solution
A buffer solution resists a change in pH on the addition of a small amount of acid or base.
An acidic buffer is a mixture of a weak acid and its salt with a strong base; a basic buffer is a mixture of a weak base and its salt with a strong acid.
$HNO_2$ is a weak acid and $NaNO_2$ is its salt with the strong base NaOH, so this pair constitutes an acidic buffer.
Added $H^+$ is removed by $NO_2^-$ ($NO_2^- + H^+ \rightarrow HNO_2$), and added $OH^-$ is removed by $HNO_2$ ($HNO_2 + OH^- \rightarrow NO_2^- + H_2O$).
NaOH is a strong base, and $HNO_3$ and HCl are strong acids; a strong acid or strong base with its salt does not form a buffer.
So the buffer is $HNO_2$ and $NaNO_2$.
An acidic buffer is a mixture of a weak acid and its salt with a strong base; a basic buffer is a mixture of a weak base and its salt with a strong acid.
$HNO_2$ is a weak acid and $NaNO_2$ is its salt with the strong base NaOH, so this pair constitutes an acidic buffer.
Added $H^+$ is removed by $NO_2^-$ ($NO_2^- + H^+ \rightarrow HNO_2$), and added $OH^-$ is removed by $HNO_2$ ($HNO_2 + OH^- \rightarrow NO_2^- + H_2O$).
NaOH is a strong base, and $HNO_3$ and HCl are strong acids; a strong acid or strong base with its salt does not form a buffer.
So the buffer is $HNO_2$ and $NaNO_2$.