Question

Buffer capacity is maximum when:

• A. One mole of ${NH}_{4}Cl$ is added to two moles of ${NH}_{4}OH$
• B. One mole of ${NH}_{4}Cl$ is added to one mole of ${NH}_{4}OH$
• C. One mole of ${NH}_{4}Cl$ is added to one moles of ${NH}_{4}OH$
• D. One mole of $NaCl$ is added to one moles of ${NH}_{4}OH$

Answer 1

Answer: (B)

One mole of ${NH}_{4}Cl$ is added to one mole of ${NH}_{4}OH$

Buffer capacity of a solution is maximum under the following conditions :

[Salt] $=$ [Acid] (in acid buffer)

[Salt] $=$ [Base] (in base buffer)

In this condition : $pH={pK}_a$

or $pOH={pK}_b$

Now the conditions $A$ and $C$ involve $\left[{NH}_{4}Cl\right]\ne \left[{NH}_{4}OH\right]$.

Only the condition $B$ where $\left[{NH}_{4}Cl\right]=\left[{NH}_{4}OH\right]$ has maximum buffer capacity.