Q24 / 160 · AP EAMCET 2005 · Chemistry

Assertion (A) The $\mathrm{pH}$ of a buffer solution containing equal moles of acetic acid and sodium acetate is 4.8 ( $\mathrm{p} K_a$ of acetic acid is 4.8). Reason (R) The ionic product of water at $25^{\circ} \mathrm{C}$ is $10^{-14} \mathrm{~mol}^2 . \mathrm{L}^{-2}$. The correct answer is

  • A
    Both (A) and (R) are true and (R) is the correct explanation of (A)
  • Correct answer
    B
    Both (A) and (R) are true and (R) is not the correct explanation of (A)
  • C
    (A) is true but (R) is not true
  • D
    (A) is not true but (R) is true

Solution

Both (A) and (R) are true statement but (R) is not the correct explanation of (A). $\mathrm{pH}$ of a buffer solution is given by Henderson-Haselbalch equation. $\begin{aligned} & \mathrm{pH}=\mathrm{p} K_a+\log \frac{[\text { salt }]}{\text { [acid }]} \\ & \therefore \quad \mathrm{pH}=\mathrm{p} K_a(\text { when }[\text { salt }]=[\text { acid }]) \\ & \end{aligned}$

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

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