Question

Assertion: PH of a buffer solution does not change on dilution.
Reason: On dilution, the ratio of concentration of salt and acid (or base) remains unchanged.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B. Both Assertion and Reason are correct and Reason is not the correct explanation for Assertion.
C. Assertion is correct but Reason is incorrect.
D. Assertion is incorrect but Reason is correct.

Answer 1

Buffer solution can be defined as a solution, which opposes or withstand any change in pH even when a small amount of acids or bases are added to it. PH of a buffer solution can be explained by using the Henderson-Hasselbalch equation.

Complete step by step answer:
We have learnt about the concept of buffer solutions that their pH does not change on dilution. Buffer solutions are of two types namely solutions of single substance and solution of mixtures.
Solution of a single substance basically a solution of the salt of weak acid and a base i.e. ammonium acetate $\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{COON}}{{\rm{H}}_{\rm{4}}}} \right)$. On the other hand, solutions of mixtures are further classified as acidic and basic buffers. Acidic buffer is a mixture of weak acid and a salt of this weak acid with a strong base i.e. acetic acid and sodium acetate $\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{COOH}} + {\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{COONa}}} \right)$ while basic buffer is a mixture of weak base and a salt of this weak base with a strong acid i.e. ammonium hydroxide and ammonium chloride $\left( {{\rm{N}}{{\rm{H}}_{\rm{4}}}{\rm{OH}} + {\rm{N}}{{\rm{H}}_{\rm{4}}}{\rm{Cl}}} \right)$.
Now, the pH of the acidic buffer solution (salt of weak acid and strong base) can be easily described by Henderson-Hasselbalch equation is as follows:
${\rm{pH}} = {\rm{p}}{{\rm{k}}_{\rm{a}}} + \dfrac{{\left[ {{\rm{Salt}}} \right]}}{{\left[ {{\rm{Acid}}} \right]}}$
Here, $\dfrac{{\left[ {{\rm{Salt}}} \right]}}{{\left[ {{\rm{Acid}}} \right]}}$ is a concentration ratio, which does not change on dilution.
Here, both Assertion and Reason are correct and Reason is the correct explanation for Assertion
So, the correct answer is “Option A”.

Note:
Some important points about buffer solutions need to be remembered that pH of a buffer solution also does change on dilution or on keeping the solution for a long period. pH of a buffer solution changes with temperature because $K_{w}$ changes (pH decreases on increasing temperature).