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JEE Advanced levelSingle correct optionDifficulty 3 of 3g2051

Chloroacetic acid has $p K_{a} = 2.86$ at 298 K. The pH of a 0.10 M aqueous solution, calculated with the exact quadratic expression, is closest to

A2.43
B1.96
C2.86
D1.43

Answer: B

K_{a} = 1 0^{- 2.86} = 1.380 e - 03

. Solving

x^{2} / (0.10 - x) = K_{a}

gives

x = [H X^{+}] = 1.108 e - 02 M

, so pH

= 1.96

. The value 1.43 would result from assuming complete ionisation, and 2.86 is simply the pKa.

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