If `pK_(b)` for fluoride ion at `25^(@)C` is 10.83 the ionisation constant of hydrofluoric acid at this temperature is

Options

A. `1.74xx10^(-5)`
B. `3.52 xx 10^(-3)`
C. `6.75 xx 10^(-4)`
D. `5.38xx10^(-2)`.

Answer

C

Explanation

The dissociation constants of a weak acid and its conjugate base are related as
` pK_(a)+pK_(b)=pK_(w)`
`:. pK_(a)+10.83=14 "or " pK_(a)=3.17`
`- log K_(a)=3.17`
or log `K_(a)=-3.17=bar(4).83 "or" K_(a)=6.76xx10^(-4)`.