Question

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

• A. $ H_{3}PO_{4},H_{2}PO_{4}^{-} $
• B. $ H_{2}PO_{4}^{-},PO_{4}^{3-} $
• C. $ HPO_{4}^{2-},PO_{4}^{3-} $
• D. $ H_{3}PO_{4},\,PO_{4}^{3-} $

Answer 1

Answer: (B)

$H _{2} PO _{4}^{-}+ H _{2} OHPO _{4}^{2-}+ \underset{\text{acid}}{H _{3} O ^{+}}$
$ H _{2} PO _{4}^{-}+ H _{2} OH _{3} PO _{4}+\underset{\text { base }}{ OH ^{-}}$
Similarly, $HPO _{4}^{2-}+ H _{2} OPO _{4}^{3-}+\underset{\text { acid }}{ H ^{+}} HPO _{4}^{2-}+ H _{2} OH _{2} PO _{4}^{-}+\underset{\text { base }}{ OH ^{-}}$
$\therefore H _{2} PO _{4}^{-}$ and $HPO _{4}^{2-}$ are amphiprotic anions which can undergo ionisation to form $H^{+}$ions and can undergo hydrolysis to form $O H^{-}$ions.
Thus, for preparing a buffer solution of $p H=7.0$,
the buffer system of weak acid $H _{2} PO _{4}^{-}$ and its conjugated base $H O _{4}^{2-}$ are used.