At what pH will a `1xx10^(-4)M` solution of an acid base indicator H In will change its colour ?
`(K_(b)"for" In^(-)=10^(11))`

Options

A. `7.0`
B. `3.0`
C. `5.5`
D. `9.0`

Answer

B

Explanation

`H In hArr H^(+)+In^(-)`
`K_(a)= ([H^(+)][In^(-)])/([H In])`
`:. K_(a)= (K_(w))/(K_(b))= (10^(-14))/(10^(-11))= [H^(+)]`
`:' [In^(-)]= [H In]`
or `[H^(+)]=10^(-3)`
or `pH=3`