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_____________.

Answer 1

Step 1: Given data

• ${\mathrm{pk}}_{\mathrm{a}}$ of phosphoric acid$=5.23$
• ${\mathrm{pk}}_{\mathrm{b}}$ of ammonium hydroxide$=4.75$

Step 2: Compute the $pH$ of ammonium phosphate

• 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 $\left({\mathrm{NH}}_4\mathrm{OH}\right)$.So,
• $\begin{array}{rcl}\mathrm{pH}& =& 7+\frac{1}{2}\left({\mathrm{pk}}_{\mathrm{a}}-{\mathrm{pk}}_{\mathrm{b}}\right)\\ \mathrm{pH}& =& 7+\frac{1}{2}\left(5.23-4.75\right)\\ \mathrm{pH}& =& 7.24\end{array}$

The $pH$ of ammonium phosphate solution will be approximately $7$