Question

5 moles of $N{H}_{4}OH$ and 1 mole of HCl are mixed and diluted to 1 litre. $pKb$ of $N{H}_{4}OH$ is 4.8. The $pH$ of the solution is:

• A. $4.8$
• B. $4.1979$
• C. $9.8021$
• D. $9.2$

Answer 1

Answer: (C)

$9.8021$

1 mole of HCl react with 1 mole of ammonium hydroxide to form 1 mole of ammonium chloride. 4 moles of ammonium hydroxide remain unreacted.
Thus the solution obtained is basic buffer solution.

$\left[salt\right]=1$ M and $\left[base\right]=4$ M.

The expression for the pOH of the basic buffer solution is as given below.

$pOH=p{K}_b+log\frac{\left[salt\right]}{\left[base\right]}$

But $p{K}_b=4.8$.

Substitute values in the above equation.

$pOH=4.8+log\left(\frac{1}{4}\right)=4.1979$

$pH=14-pOH=14-4.1979=9.8021$.

Option C is correct.