When glycinium hydrochloride () is titrated against NaOH, pH at the first half equivalence point is — Ionic Equilibrium Chemistry Question
Question
When glycinium hydrochloride ($\text{NH}_2\text{CH}_2\text{COOH}\cdot\text{HCl}$) is titrated against NaOH, pH at the first half equivalence point is 2.40 and the pH at second half equivalence point is 9.60. The pH at first equivalence point is
💡 Solution & Explanation
Glycinium hydrochloride behaves as a diprotic acid ($\text{H}_2\text{A}^+$). At the first half equivalence point, $\text{pH} = pK_{a1} = 2.40$. At the second half equivalence point, $\text{pH} = pK_{a2} = 9.60$. The first equivalence point corresponds to the formation of the intermediate amphiprotic species ($\text{HA}$, which is the zwitterion form of glycine). The pH is given by the average of the two $pK_a$ values: $\text{pH} = \frac{pK_{a1} + pK_{a2}}{2} = \frac{2.40 + 9.60}{2} = \frac{12.00}{2} = 6.00$. Therefore, correct answer is C.