(a)[1]
Write the $K_a$ expression for $\text{H}_2\text{PO}_4^{-}$.
(b(i))[2]
Explain what the term buffer solution means.
(b(ii))[2]
Write two equations to demonstrate how a mixture of $\text{H}_2\text{PO}_4^{-}$ and $\text{HPO}_4^{2-}$ functions as a buffer.
(c)[3]
Calculate the value of $[\text{HPO}_4^{2-}]/[\text{H}_2\text{PO}_4^{-}]$ needed to produce a pH of $7.40$ in the cells, given that $K_a = 6.31 \times 10^{-8}\,\text{mol dm}^{-3}$.
(d(i))[1]
Write an equation to show $\text{H}_2\text{PO}_4^{-}$ acting as a base.
(d(ii))[1]
Write an equation to show $\text{HPO}_4^{2-}$ acting as an acid.