91Ó°ÊÓ

A buffer solution can withstand pH changes when acidic or basic components are added. It can neutralize little amounts of additional acid or base, allowing the pH of the solution to remain relatively constant.

To find the buffer components, first create the dissolving equation for each salt.

$$\begin{gathered} {\text{KH}}_{\text{2}}{\text{PO}}_{\text{4}}→{\text{K}}^{\text{+}}{\text{+H}}_{\text{2}}{\text{PO}}_{\text{4}}^{\text{-}} \\ {\text{Na}}_{\text{2}}{\text{HPO}}_{\text{4}}→{\text{2Na}}^{\text{+}}{\text{+ HPO}}_{\text{4}}^{\text{2 -}} \end{gathered}$$

As${\text{H}}_{\text{2}}{\text{PO}}_{\text{4}}^{\text{-}}$ and${\text{HPO}}_{\text{4}}^{\text{2 -}}$ are the ions that will be involved in the buffer reaction.

${\text{H}}_{\text{2}}{\text{PO}}_{\text{4}}^{\text{-}}{\text{+H}}_{\text{2}}\text{O}⇌{\text{HPO}}_{\text{4}}^{\text{2 -}}{\text{+H}}_{\text{3}}{\text{O}}^{\text{+}}$

Calculate the from${\text{pK}}_{\text{a}}$ the${\text{K}}_{\text{a}}$ of above reaction in Appendix C.

$$\begin{gathered} {\mathrm{pK}}_{\mathrm{a}}=-{\mathrm{logK}}_{\mathrm{a}} \\ =-\log \left(6.3×{10}^{-8}\right) \\ {\mathrm{pK}}_{\mathrm{a}}=7.20 \end{gathered}$$

Solve for the concentration ratio$\left(\frac{\left[{\text{KH}}_{\text{2}}{\text{PO}}_{\text{4}}\right]}{\left[{\text{Na}}_{\text{2}}{\text{HPO}}_{\text{4}}\right]}\right)$ using the Henderson-Hassle Balch equation.

$$\begin{gathered} {\text{pH = pK}}_{\text{a}}\text{+ log}\frac{\left[{\text{KH}}_{\text{2}}{\text{PO}}_{\text{4}}\right]}{\left[{\text{Na}}_{\text{2}}{\text{HPO}}_{\text{4}}\right]} \\ \text{log}\frac{\left[{\text{KH}}_{\text{2}}{\text{PO}}_{\text{4}}\right]}{\left[{\text{Na}}_{\text{2}}{\text{HPO}}_{\text{4}}\right]}\text{= pH - pKa} \end{gathered}$$

$$\begin{gathered} \text{log}\frac{\left[{\text{KH}}_{\text{2}}{\text{PO}}_{\text{4}}\right]}{\left[{\text{Na}}_{\text{2}}{\text{HPO}}_{\text{4}}\right]}\text{= 7.4 - 7.20} \\ \frac{\left[{\text{KH}}_{\text{2}}{\text{PO}}_{\text{4}}\right]}{\left[{\text{Na}}_{\text{2}}{\text{HPO}}_{\text{4}}\right]}{\text{= 10}}^{\text{7.4 - 7.20}} \\ \frac{\left[{\text{KH}}_{\text{2}}{\text{PO}}_{\text{4}}\right]}{\left[{\text{Na}}_{\text{2}}{\text{HPO}}_{\text{4}}\right]}\text{= 1.58} \end{gathered}$$

Therefore, $\frac{\left[{\text{KH}}_{\text{2}}{\text{PO}}_{\text{4}}\right]}{\left[{\text{Na}}_{\text{2}}{\text{HPO}}_{\text{4}}\right]}\text{= 1.58}$.