Which of the following mixtures in aqueous solution acts as a buffer?
The correct option is A NH4OH+NH4Cl
Solution which resist any change in pH on dilution or with the addition of an appreciable amount of an acid or alkali is known as buffer solution.
Option (a) and (b) contain strong acid and salt of strong acid and strong base thus it does not form buffer solution.
(c) A mixture of ammonium hydroxide and HCl react to form ammonium chloride. This also contains unreacted ammonium hydroxide.
Thus, the resulting mixture of (NH4OH+NH4Cl) is a basic buffer solution. It contains a mixture of weak base(NH4OH) and its salt (NH4Cl) with a strong acid (HCl).
Option (d) contains a mixture of weak acid (CH3COOH) and salt of strong acid and strong base (NaOH) thus it also does not form a buffer solution.