Given: $K_{\mathrm{a}}\left({\mathrm{H}}_2{\mathrm{PO}}_4^{-}\right)=6.28\times {10}^{-8} \mathrm{M}$. A buffer is made by mixing ${\mathrm{NaH}}_2{\mathrm{PO}}_4$ and  ${\mathrm{Na}}_2{\mathrm{HPO}}_4$.The range of $\mathrm{pH}$ to which this buffer can be used is

$\mathrm{p}{K}_{\mathrm{a}}^{\circ }=-\log \left(6.28\times {10}^{-8}\mathrm{M}\right)=7.21$          Buffer range is $\mathrm{p}{K}_{\mathrm{a}}^{\circ }-1\text{ to }\mathrm{p}{K}_{\mathrm{a}}^{\circ }+1$.

The pH range that a buffer may be used effectively is one pH unit on each side of the weak acid's ${\mathrm{pk}}_{\mathrm{a}}$ value. A buffer loses its ability to withstand significant pH variations if the pH is outside of this range.