The $\mathrm{pK}_{\mathrm{a}}$ of a weak acid, HA, is 4.80. The $\mathrm{pK}_{\mathrm{b}}$ of a weak base, BOH, is 4.78. The pH of an aqueous solution of the corresponding salt, BA, will be

JEE Main · 2008 · 27 Apr 2008 · Offline · Equilibrium

The

pK_{a}

of a weak acid, HA, is 4.80. The

pK_{b}

of a weak base, BOH, is 4.78. The pH of an aqueous solution of the corresponding salt, BA, will be

Answer: (C)

For the salt of a weak acid and a weak base the hydrolysis of the two ions very nearly cancels, and the pH is given by

pH = 7 + \frac{1}{2} (p K_{a} - p K_{b})

. With

p K_{a} = 4.80

and

p K_{b} = 4.78

the bracket is

0.02

, so

pH = 7 + \frac{1}{2} (0.02) = 7.01

, very slightly on the basic side of neutral.

JEE Main · 2008 · 27 Apr 2008 · Offline · Single correct option