Identify the type of titration

We are titrating a weak base $\mathrm{NH_4OH}$ with a strong acid $\mathrm{HCl}$.

So initially, the solution is basic, but not very strongly basic because $\mathrm{NH_4OH}$ is a weak base.

What happens during titration?

The reaction is: $\mathrm{NH_4OH + HCl \rightarrow NH_4Cl + H_2O}$

As HCl is added:

• pH decreases gradually at first.
• A buffer region is formed containing weak base and its salt.
• At equivalence point, the solution contains mainly $\mathrm{NH_4Cl}$, which is a salt of a weak base and strong acid.

Nature of the equivalence point

Since $\mathrm{NH_4Cl}$ undergoes hydrolysis: $\mathrm{NH_4^+ + H_2O \rightleftharpoons NH_3 + H_3O^+}$ the solution at equivalence is acidic.

Therefore: $\text{pH at equivalence point} < 7$

Shape of the titration curve

For a weak base–strong acid titration:

• Initial pH is above 7, but not very high.
• The curve falls gradually.
• The sharp fall near equivalence is less steep than strong base–strong acid titration.
• The equivalence point lies below 7.
• After equivalence, excess strong acid makes pH very low.

Matching with the correct graph

Hence the correct plot must:

• start in the basic region,
• decrease as acid is added,
• have equivalence point below 7.

This corresponds to Option A.

Comparison with stored answer

Stored correct answer = A.

Our derived answer = A.

So they agree.