Question

Which of the following mixtures will be buffers?
A ) \[{\text{C}}{{\text{H}}_3}{\text{COOH + C}}{{\text{H}}_3}{\text{COON}}{{\text{H}}_4}\]
B ) \[{\text{HCl + NaCl}}\]
C ) Borax + boric acid
D ) \[{\text{C}}{{\text{H}}_3}{\text{COOH + C}}{{\text{H}}_3}{\text{COONa}}\]

Answer 1

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

Complete answer:
Buffer solutions have the ability to resist the pH of the solution. When we add a small amount of weak acid or weak base, there is no change in the pH. When there is addition of a small amount of strong acid, the hydrogen ions from strong acid combine with the conjugate base of the buffer solution to form feebly ionized weak acid. Similarly when there is addition of a small amount of strong base to a buffer solution, the hydroxide ions from the strong base combine with the weak acid of the buffer solution to form conjugate base and water.
Let us discuss from the given mixtures which will act as a buffer:
A ) \[{\text{C}}{{\text{H}}_3}{\text{COOH + C}}{{\text{H}}_3}{\text{COON}}{{\text{H}}_4}\] is a mixture of weak acid (acetic acid) and its salt ammonium acetate with weak base (ammonia). Hence, it is not a buffer solution.
B ) \[{\text{HCl + NaCl}}\] is a mixture of strong acid (hydrochloric acid) and its salt sodium chloride with strong base (sodium hydroxide). Hence, it is not a buffer solution.
C ) Borax + boric acid is a mixture of weak acid (boric acid) and its salt borax with strong base (sodium hydroxide). Hence, it is not a buffer solution.
D ) \[{\text{C}}{{\text{H}}_3}{\text{COOH + C}}{{\text{H}}_3}{\text{COONa}}\] is a mixture of weak acid (acetic acid) and its salt sodium acetate with strong base (sodium hydroxide). Hence, it is an acid buffer solution.

Hence, the option D) is the correct option.

Note: A buffer solution resists change in pH. When a small amount of an acid (or a base) is added to the buffer solution, the pH of the solution remains unchanged.