Question

$1M \, N H_{4}OH$ and $1M \, HCl$ are mixed to make a total volume of $300 \, mL$ . If $pH$ of the mixture is $9.26$ and $pK_{a}\left(N H_{4}^{+}\right)=9.26$ then what would be the volume ratio of $N H_{4}OH$ and $HCl$ .

• A. 2:1
• B. 1:2
• C. 2:3
• D. 3:2

Answer 1

Answer: (A)

$pOH =14- pH = pK _{ b }+\log \frac{ x }{300-2 x }$
$=14-9.26=-\log \left(10^{-4.74}\right)+\log \frac{ x }{300-2 x }$
$\Rightarrow \, \frac{x}{300 - 2 x}=1$
$\Rightarrow 300=3x\Rightarrow x=100 \, mL$
$\frac{\text{V}_{\text{NH}_{\text{4}} \text{OH}}}{\text{V}_{\text{HCl}}} \, \text{=} \, \text{2}$