The pKa values of 4-nitrobenzoic acid and acetic acid in water are 3.44 and 4.76 respectively. Calculate the ratio $K_{a} (4-nitrobenzoic acid) / K_{a} (acetic acid)$ . Give the answer to the nearest integer.

Numerical answer type, tolerance 0.5.

Answer: 21

Since

p K_{a} = - lo g K_{a}

, the ratio is

1 0^{Δ p K_{a}}

. Here

Δ p K_{a} = 4.76 - 3.44 = 1.32

, so the ratio is

1 0^{1.32} = 20.89

, i.e. about 21.