Which of the following mixtures will produce a buffer solution when mixed in equal volumes?

1. 0.1 mol dm-3 NH4OH and 0.1 mol dm-3 HCl

2. 0.05 mol dm-3 NH4OH and 0.1 mol dm-3 HCl

3. 0.1 mol dm-3 NH4OH and 0.05 mol dm-3 HCl

4. 0.1 mol dm-3 CH3COONa and 0.1 mol dm-3 NaOH

Hint: Weak base and its salt with strong acid forms buffer when both are present together in solution
Explanation:
Step 1:
When the concentration of ${\mathrm{NH}}_{4}\mathrm{OH}$ (a weak base) is higher than the strong acid (HCl), a mixture of a weak base
and its conjugate acid is obtained, which acts as a basic buffer.
Step 2:
The reaction is as follows:
In the solution, the weak base and its conjugate acid are left. It is an example of a buffer. Hence, option third is the correct answer.