Question

For preparing a buffer solution of $pH=7.0$, which buffer system you will choose?

• A. $H_3{PO}_4,H_2{PO}_4^{-}$
• B. $H_2{PO}_4^{-},{HPO}_4^{2-}$
• C. ${HPO}_4^{2-},{PO}_4^{3-}$
• D. $H_3{PO}_4,{PO}_4^{3-}$

Answer 1

Answer: (B)

$H_2{PO}_4^{-},{HPO}_4^{2-}$

Using ${H_{2}P{O}_4}^{-}$, ${HP{O}_4}^{-2}$, a buffer solution of $pH=7$ is prepared.

The equilibrium reaction involved in this solution will be dissociation of ${H_{2}P{O}_4}^{-}$ in water.

${H_{2}P{O}_4}^{-}+H_{2}O\rightleftharpoons {HP{O}_4}^{2-}+{H_{3}O}^{+}$

For phosphate buffer, $p{K}_a$ value of ${H_{2}P{O}_4}^{-}$ is equal to $7.2$ so that buffer system is suitable for $pH$ range $7.2\pm 1$ or from $6.2$ to $8.2$