Question

The pKa of benzoic acid (C6H5COOH) is 4.19. This means that

A. in a solution at pH = 7.0, benzoic acid would be nearly or completely protonated.

B. in a solution at pH = 7.0, benzoic acid would be nearly or completely deprotonated.

C. in a solution at pH = 4.19, benzoic acid would be 1/2 deprotonated (1/2 benzoic acid and 1/2 benzoate).

A and C

B and C

Answer 1

Correct option: B and C

From Henderson-Hasselbalch equation,

pH = pKa + log([benzoate]/[benzoic acid])

or, 7.0 = 4.19 + log([benzoate]/[benzoic acid])

or, log([benzoate]/[benzoic acid]) = 2.81

or, [benzoate]/[benzoic acid] = 645

or, [benzoate] = 645 x [benzoic acid]

Thus, at pH 7.0, the concentration of benzoate in the solution is 645 times higher than the concentration of benzoic acid.

Hence, in a solution at pH = 7.0, benzoic acid would be nearly or completely deprotonated.

Thus, option B is correct and option A is incorrect.

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At pH = pK_a, half of the acid is deprotonated.   Hence, option C is correct.