Question

Which of the following is a buffer solution?
(A) ${\text{NaOH + C}}{{\text{H}}_{\text{3}}}{\text{COONa}}$
(B) ${\text{NaOH + N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_4}$
(C) ${{\text{K}}_{\text{2}}}{\text{S}}{{\text{O}}_4}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_4}$
(D) ${\text{N}}{{\text{H}}_4}{\text{OH + N}}{{\text{H}}_4}{\text{Cl}}$

Answer 1

An acid buffer solution is the mixture of weak acid and its conjugate base. An acid buffer solution is the mixture of weak acid and its salt with strong base. A basic buffer solution is the mixture of weak base and its conjugate acid. A basic buffer solution is the mixture of weak base and its salt with strong acid.

Complete answer:
Let us consider the given options one by one:
(A) ${\text{NaOH + C}}{{\text{H}}_{\text{3}}}{\text{COONa}}$ is not a buffer solution. Sodium hydroxide is a strong base. Sodium acetate is a salt of weak acid (acetic acid) with a strong base (sodium hydroxide).
(B) ${\text{NaOH + N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_4}$ is not a buffer solution. Sodium hydroxide is a strong base. Sodium sulphate is a salt of strong acid (sulphuric acid) with a strong base (sodium hydroxide).
(C) ${{\text{K}}_{\text{2}}}{\text{S}}{{\text{O}}_4}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_4}$ is not a buffer solution. Potassium sulphate is a salt of strong acid (sulphuric acid) with a strong base (potassium hydroxide). Sulphuric acid is a strong acid.
(D) ${\text{N}}{{\text{H}}_4}{\text{OH + N}}{{\text{H}}_4}{\text{Cl}}$ is a basic buffer solution. Ammonium hydroxide is a weak base. Ammonium chloride is a salt of weak base (ammonia or ammonium hydroxide) with strong acid (hydrochloric acid).

Additional Information: When an acid buffer solution has equal number of moles of weak acid and its conjugate base, the pH of the solution is equal to its ${\text{p}}{{\text{K}}_a}$ value.
${\text{p}}{{\text{K}}_a}$ is equal to the negative logarithm of hydrogen ion concentration.

Note: Henderson Hasselbach equation gives the expression for the pH of the buffer solution. For an acid buffer solution, the pH of the solution is given by the following expression:
${\text{pH = p}}{{\text{K}}_a}{\text{ + log}}\dfrac{{\left[ {{\text{ conjugate base }}} \right]}}{{\left[ {{\text{ acid }}} \right]}}$