25 ml? — Ionic Equilibrium Chemistry Question

💡 Solution & Explanation

Adding $25\text{ ml}$ of $1.0\text{ M HCl}$ ($25\text{ mmol}$) to $50\text{ mmol}$ of $\text{Na}_2\text{CO}_3$ neutralizes exactly half of the $\text{CO}_3^{2-}$, converting it into $\text{HCO}_3^-$. This results in a buffer solution containing equimolar amounts of $\text{CO}_3^{2-}$ and $\text{HCO}_3^-$. According to the Henderson-Hasselbalch equation for this step, $\text{pH} = pK_{a2}$. Given $K_{a2} = 2.5 \times 10^{-11}$, $pK_{a2} = 11 - \log(2.5) = 11 - 0.4 = 10.6$. Therefore, correct answer is A.