Q.Equilibrium :
Assertion (A) : A solution containing a mixture of ammonium chloride and ammonium hydroxide maintains a constant value of pH on addition of small amounts of acid or alkali.
Reason (R) : A solution containing mixture of ammonium chloride and ammonium hydroxide act as a buffer solution around pH 9.25.
(Choose the correct option for the Assertion-Reason pair.)

Solutions that resist a change in pH on dilution (or on adding small amounts of acid/base) are called buffer solutions.

A buffer solution typically consists of a weak acid together with its conjugate base (e.g. CH3COOH + CH3COO- from CH3COONa), or a weak base together with its conjugate acid (e.g. NH4OH + NH4+ from NH4Cl). Because both the acidic and basic components are present in appreciable, comparable amounts, any small addition of H+ or OH- (or dilution, which shifts conce …

Both the assertion and the reason are true, and the reason correctly explains the assertion: NH4Cl + NH4OH is a basic buffer solution with pH close to 9.25.

A buffer solution resists changes in pH upon addition of small amounts of acid or base. A basic buffer is typically made of a weak base and its salt with a strong acid — exactly the NH4OH (weak base) + NH4Cl (its salt with strong acid HCl) combination described here.

How the buffering works: NH4OH partially ionises to give NH4+ and OH-, while NH4Cl fully dissociates to give a large reservoir of NH4+ ions and Cl- ions.

• On adding a small amount of acid (H+), the excess H+ is neutralised by the NH4OH present (NH4OH + H+ → NH4+ + H2O), so pH barely changes.
• On adding a small amount of base (OH-), the NH4+ from the salt neutralises it (NH4+ + OH- → NH4OH), again keeping pH nearly constant. …

A buffer resists pH change on adding small amounts of acid/base; made of a weak acid + its conjugate base (or weak base + its conjugate acid).

A buffer solution is a solution whose pH changes very little when a small amount of a strong acid or strong base is added to it, or on dilution. It is typically prepared by mixing:

• a weak acid with a salt of its conjugate base (an acidic buffer, e.g. CH3COOH + CH3COONa), or
• a weak base with a salt of its conjugate acid (a basic buffer, e.g. NH4OH + NH4Cl). …

The Henderson-Hasselbalch equation: pH = pKa + log([salt]/[acid]), used to calculate the pH of a buffer solution.

For a buffer made of a weak acid (HA) and its conjugate base/salt (A-, e.g. from a sodium salt), the equilibrium HA <-> H+ + A- gives Ka = [H+][A-]/[HA].

…

A buffer solution maintains an almost constant pH despite small additions of acid, base, or dilution, because it contains a weak acid/conjugate-base or weak base/conjugate-acid pair that can absorb added H+ or OH-.

A buffer solution resists changes in pH upon the addition of small quantities of an acid or a base, or upon dilution. There are two main types:

1. Acidic buffer: a mixture of a weak acid and its salt with a strong base (e.g., CH3COOH + CH3COONa). It resists pH changes near acidic pH values.
2. Basic buffer: a mixture of a weak base and its salt with a strong acid (e.g., NH4OH + NH4Cl). It resists pH changes near basic pH values. …