JEE Main · 2007 · 29 Apr 2007 · Offline · Equilibrium

The pKa of a weak acid (HA) is 4.5. The pOH of an aqueous buffered solution of HA in which

50%

of the acid is ionized is

Answer: (C)

Solution

For a buffer the Henderson equation gives

pH = p K_{a} + lo g \frac{[ salt ]}{[ acid ]}

. When the acid is half ionised the salt and acid concentrations are equal, the logarithm is zero, and the pH equals the

p K_{a}

, which is

4.5

. Since

pH + pOH = 14

at this temperature, the pOH is

14 - 4.5 = 9.5

JEE Main · 2007 · 29 Apr 2007 · Offline · Single correct option