Find the pH of the mixture obtained by mixing of and of . (Given ) — Ionic Equilibrium Chemistry Question
Question
Find the pH of the mixture obtained by mixing $50 \text{ mL }$ of $0.2 \text{ M } H_3PO_4$ and $50 \text{ mL }$ of $0.2 \text{ M } Na_3PO_4$. (Given $pK_{a1} = 2.12, pK_{a2} = 7.2, pK_{a3} = 12.0$)
💡 Solution & Explanation
Millimoles of $H_3PO_4 = 50 \times 0.2 = 10 \text{ mmol}$. Millimoles of $Na_3PO_4 = 50 \times 0.2 = 10 \text{ mmol}$. The extreme acidic and basic species react completely (comproportionation): $H_3PO_4 + PO_4^{3-} \rightarrow H_2PO_4^- + HPO_4^{2-}$. Since both reactants are exactly $10 \text{ mmol}$, they are completely consumed, producing exactly $10 \text{ mmol}$ of $H_2PO_4^-$ and $10 \text{ mmol}$ of $HPO_4^{2-}$. This forms an ideal buffer system consisting of the second dissociation pair ($H_2PO_4^- / HPO_4^{2-}$). Since their concentrations are perfectly equal, $pH = pK_{a2} + \log(1) = pK_{a2}$. Therefore, $pH = 7.2$.