Q20 / 160 · AP EAMCET 2020 (18 Sep Shift 2) · Chemistry

The \(\mathrm{p} K_a\) of a weak acid, benzoic acid and \(\mathrm{p} K_b\) of a weak base, ammonium hydroxide are 4.25 and 4.75 respectively. Then the \(\mathrm{pH}\) of \(0.1 \mathrm{M}\) solution of ammonium benzoate will be

  • A
    7.10
  • B
    7.50
  • Correct answer
    C
    6.75
  • D
    6.50

Solution

Ammonium benzoate \(\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{COONH}_4\right)\) is the salt weak benzoic acid \(\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{COOH}, \mathrm{p} K_a=4.25\right)\) and weak ammonium hydroxide \(\left(\mathrm{NH}_4 \mathrm{OH}, \mathrm{p} K_b=4.75\right)\). So, \(\mathrm{pH}\) of the solution of \(\mathrm{C}_6 \mathrm{H}_5 \mathrm{COONH}_4\) will be, \(\begin{aligned} \mathrm{pH} & =7+\frac{1}{2}\left(\mathrm{p} K_a-\mathrm{p} K_b\right) \\ & =7+\frac{1}{2}(4.25-4.75)=7-0.25=6.75 \end{aligned}\)

Hence, the correct answer is option: 'C'.

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