Q14 / 90 · KVPY 2012 (SB/SX) · Chemistry

The $\mathrm{pK}_{\mathrm{a}}$ of a weak acid is $5.85$. The concentrations of the acid and its conjugate base are equal at a $\mathrm{pH}$ of $-$

A
$6.85$
Correct answer
B
$5.85$
C
$4.85$
D
$7.85$

Solution

$\mathrm{pH}=\mathrm{p} \mathrm{Ka}+\log \frac{[\mathrm{Conjugate} \text { base] }}{[\text { Acid }]}$ $\because[$ Conjugate base $]=[$ Acid $]$ $\mathrm{pH}=\mathrm{pKa}=5.85$

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

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