75 ml? — Ionic Equilibrium Chemistry Question

💡 Solution & Explanation

Adding $75\text{ ml}$ of $1.0\text{ M HCl}$ ($75\text{ mmol}$) converts all $50\text{ mmol}$ of $\text{Na}_2\text{CO}_3$ into $\text{NaHCO}_3$, and the remaining $25\text{ mmol}$ of HCl converts half of the $\text{NaHCO}_3$ into $\text{H}_2\text{CO}_3$. This creates an acidic buffer containing $25\text{ mmol}$ of $\text{HCO}_3^-$ and $25\text{ mmol}$ of $\text{H}_2\text{CO}_3$. The pH is governed by the first dissociation step: $\text{pH} = pK_{a1} = 5.4$. Therefore, correct answer is D.