25.0 mL of a mixture that is 0.10 M in HCl and 0.10 M in CH3COOH is titrated with 0.10 M NaOH (K_a = 1.8x10^-5 for acetic acid). At the first end point, where all the HCl has just been neutralised, the pH of the solution is closest to

JEE Advanced levelSingle correct optionDifficulty 3 of 3ia049

25.0 mL of a mixture that is 0.10 M in HCl and 0.10 M in $CH X_{3} COOH$ is titrated with 0.10 M NaOH ( $K_{a} = 1.8 \times 1 0^{- 5}$ for acetic acid). At the first end point, where all the HCl has just been neutralised, the pH of the solution is closest to

At the first end point, 25.0 mL of NaOH has been added (2.5 mmol), giving 50 mL of solution with 0.050 M

CH X_{3} COOH

and neutral NaCl. Acetic acid is then the only acid:

[H X^{+}] = K_{a} c = 9.49 e - 04

M, pH

= \frac{1}{2} (p K_{a} - lo g c) = 3.02

. The choice 1.30 wrongly assumes full ionisation of the acetic acid.