The `pK_a` 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

Options

A. `7.0`
B. `4.5`
C. `2.5`
D. `9.5`

Answer

D

Explanation

For acidic buffer, `pH=pK_a+"log" ([A^-])/([HA])`
When the acid is 50% ionised , `[A^-]=[HA]`
or pH=`pK_a+log 1` or pH = `pK_a`
Given `pK_a=4.5`
`therefore` pH=9.5
`therefore` pOH=14-4.5=9.5