If the pH of a buffer solution containing 0.1 M of monoacidic base and 0.01 M of its salt is 10.5, the `pK_(a)` of conjugate acid is

Options

A. `9.5`
B. `4.5`
C. `3.5`
D. `11.5`

Answer

B

Explanation

From Henderson-Hesselbalch equation
`pH = pK_(a) + log.(["salt"])/(["acid"])`
pH of a buffer solution = 10.5
`pK_(a)` of this buffer solution can be calculated as :
`rArr 10.5 = pK_(a) + log.([0.01])/([0.1])`
`10.5 = pK_(a) + log 10^(-1)`
`10.5 = pK_(a) - 1`
`pK_(a) = 11.5`