Question

A solution of NH₄Cl and NH₄OH acts as a buffer.

Answer 1

NH₄Cl is a strong electrolyte hence, it dissociates completely. NH₄OH is a weak electrolyte and dissociates to a small extent. Its dissociation is further suppressed by the common ion `NH_4^+`provided by NH₄Cl in the solution. 

`NH_4CI_((aq)) ⇌ NH_(4(aq))^+ + CI_max^-`

`NH_4OH_((aq)) ⇒ NH_(4(aq))^+  +OH_((aq))^-`

This solution acts as a basic buffer and maintains its pH around 9.25. It resists the change in pH on the addition of a small amount of acid or alkali. This can be explained as below:
Upon adding a small amount of HCl to this solution, H⁺ ions of HCl get neutralized by OH^– ions already present and more of NH₄OH molecules get ionized to compensate for the loss of OH^– ions. Thus, pH practically remains unchanged.
Upon adding a small amount of NaOH to this solution, OH ions of NaOH combine with `NH_4^+` ions already present to form weakly ionized NH₄OH. Thus, pH of the solution remains practically unchanged.