Questions: Construct the expression for Ka for the weak acid, HPO4^2-. HPO4^2- (aq) + H2O(l) ⇌ H3O^+ (aq) + PO4^3- (aq)

Solution Steps

The given reaction is: \[ \mathrm{HPO}_{4}^{2-}(\mathrm{aq}) + \mathrm{H}_{2}\mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{H}_{3}\mathrm{O}^{+}(\mathrm{aq}) + \mathrm{PO}_{4}^{3-}(\mathrm{aq}) \]

The acid dissociation constant \( K_a \) for a weak acid \( \mathrm{HA} \) is given by: \[ K_a = \frac{[\mathrm{H}_3\mathrm{O}^+][\mathrm{A}^-]}{[\mathrm{HA}]} \]

For the given reaction:

• The weak acid \( \mathrm{HA} \) is \( \mathrm{HPO}_4^{2-} \)
• The conjugate base \( \mathrm{A}^- \) is \( \mathrm{PO}_4^{3-} \)
• The hydronium ion \( \mathrm{H}_3\mathrm{O}^+ \) remains the same

Using the components identified: \[ K_a = \frac{[\mathrm{H}_3\mathrm{O}^+][\mathrm{PO}_4^{3-}]}{[\mathrm{HPO}_4^{2-}]} \]

Final Answer