Question

Solution of 0.1 N $N{H}_{4}OH$ and 0.1 N $N{H}_{4}Cl$ has pH 9.25. $p{K}_b$ of $N{H}_{4}OH$ is :

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

Answer 1

The correct option is B 4.75

We know the relation,
$pH+pOH=14\left(at{25}^{o}C\right)$
$9.25+pOH=14$
$pOH=14-9.25$
$pOH=4.75.....\left(1\right)$
Using the Henderson-Hasselbalch equation for weak bases:
$pOH=p{K}_b+log\left[\frac{\left[conjugateacid\right]}{\left[base\right]}\right]$
$pOH=p{K}_b+log\left(1\right)$
$pOH=p{K}_b(\because log1=0)$
$p{K}_b=4.75$ (from equation 1)
Hence $p{K}_b$ value is 4.75