Question

50 mL of 0.2 M ammonia solution is treated with 25 mL of 0.2 M HCl. If \(\mathrm{p}K_\mathrm{b}\) of ammonia solution is 4.75, the pH of the mixture will be :

• A. 3.75
• B. 4.75
• C. 8.25
• D. 9.25

Hint:

HCl neutralises half of the ammonia, giving equal moles of \(\mathrm{NH_3}\) and \(\mathrm{NH_4^+}\), a basic buffer with \(\mathrm{pOH} = \mathrm{p}K_b\).

Answer 1

The Correct Option is D

Moles of \(\mathrm{NH_3}\) = \(50\times0.2 = 10\) mmol; moles of HCl = \(25\times0.2 = 5\) mmol.
\(\mathrm{NH_3 + HCl \to NH_4Cl}\): after reaction \(\mathrm{NH_3} = 5\) mmol and \(\mathrm{NH_4^+} = 5\) mmol, a basic buffer.
\(\mathrm{pOH} = \mathrm{p}K_b + \log\frac{[\mathrm{salt}]}{[\mathrm{base}]} = 4.75 + \log\frac{5}{5} = 4.75\).
\(\mathrm{pH} = 14 - 4.75 = 9.25\).
Answer: (4) 9.25