Question

Which can act as a buffer?
A) $N{H_4}Cl + HCl$
B) $C{H_3}COOH + {H_2}C{O_3}$
C) $40ml$ of $0.1M$ $NaCN + 20ml$ of $0.1MHCl$
D) $NaCl + HCl$

Answer 1

We have to know that a weak acid is an acid that partially dissociates into its ions in an aqueous solution or water. The conjugate base of a weak acid is a weak base, while the conjugate acid of a weak base is a weak acid. At the same concentration, weak acids have a higher pH value than strong acids.

Complete answer:
As we know, a buffer is a solution of a weak acid or a base and its salt. Both components must be present for the system to act as a buffer to resist changes in pH.
We will look at all the options one by one:
Option A) this option is incorrect as buffer is a solution of weak acid and its weak salt but in this case hydrochloric acid is a strong acid thus this combination cannot be a buffer.
Option B) this option is incorrect since both are weak acids thus a buffer solution cannot be formed.
Option C) this option is correct as Sodium cyanide is a salt of weak acid and strong base while hydrogen cyanide is a weak acid. Thus they can act as a buffer solution.
Option D) this option is incorrect.

Option C is the correct answer.

Note:
We need to know that the strong acid is an acid that ionizes completely in aqueous solution. It always loses a proton $\left( {{H^ + }} \right)$ when dissolved in water. Weak acid is an acid that ionizes partially in a solution. It gives off only a few of its $\left( {{H^ + }} \right)$ atoms when dissolved in water.