An acid-base indicator which is a weak acid has a `pK_(a)` value `=5.5`. At what concentration ratio of sodium acetate to acetic acid would the indicator show a colour half-way between those of its acid and conjugate base forms?

Options

A. `4.93 : 1`
B. `6.3 : 1`
C. `5.62 : 1`
D. `2.37 : 1`

Answer

C

Explanation

At Half way `[HIn]=In`
`pH=5.5+(["salt"])/([acid])5.5`
`=pK_(a)+log``(["salt"])/([acid])`
`log``(["salt"])/([acid])=0.75implies (["salt"])/([acid])=5.62`