Question:
The $\mathrm{pH}$ of ammonium phosphate solution, if $\mathrm{pK}_{\mathrm{a}}$ of phosphoric acid and $\mathrm{pk}_{\mathrm{b}}$ of ammonium hydroxide are $5.23$ and $4.75$ respectively, is_________.
Solution:
Since $\left(\mathrm{NH}_{4}\right)_{3} \mathrm{PO}_{4}$ is salt of weak acid $\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)$ \& weak base $\left(\mathrm{NH}_{4} \mathrm{OH}\right)$.
$\mathrm{pH}=7+\frac{1}{2}(\mathrm{pka}-\mathrm{pkb})$
$=7+\frac{1}{2}(5.23-4.75)$
$=7.24 \approx 7$