Question

Solution of 0.1 N $N{H}_{4}OH$ and 0.1 N $N{H}_{4}Cl$ has pH 9.25, Then find out $p{k}_b$ of $N{H}_{4}OH$.

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

Answer 1

Answer: (B)

4.75

$N{H}_{4}OH$ (a base) and $N{H}_{4}Cl$ (a salt) undergoes dissociation as follows:-

$N{H}_{4}OH\rightleftharpoons N{H}_4^{+}+O{H}^{-}$

$N{H}_{4}Cl\to N{H}_4^{+}+C{l}^{-}$

Due to the common ion effect, the reaction of dissociation of $N{H}_{4}OH$ shifts to the left.

Here, the given information is :-

1. Concentration of salt $N{H}_{4}Cl$ = 0.1N
2. Concentration of the base $N{H}_{4}OH$=0.N
3. pH of the solution is 9.25

$\therefore$ pOH= 14 - pH = 14 - 9.25 = 4.75

We know that,

$pOH=p{K}_b+log\frac{\left[salt\right]}{\left[base\right]}$ (but here concentration of salt = concentration of base and $\left[lo{g}_{10}\right(1)=0]$)

$\therefore pOH=p{K}_b=4.75$

Hence, the correct answer is option (B).