1.

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

Answer»

`1.74 xx 10^(-5)`
`3.52 xx 10^(-3)`
`6.75 xx 10^(-4)`
`5.38 xx 10^(-2)`

Solution :`HF + H_(2)O overset(K_(a))rarr H_(3)O^(+) + F^(-)`
`:. K_(a) = ([H_(3)O^(+)][F^(-)])/([HF][H_(2)O])`...(i)
`F^(-) + H_(2)O overset(K_(b))rarr HF + OH^(-)`
`:. K_(b) = ([HF][OH^(-)])/([F^(-)][H_(2)O])`...(ii)
`:. K_(a) xx K_(b) = [H_(3)O^(+)][OH^(-)] = K_(w) = 10^(-14)`
Given, `pK_(b) = 10.83 rArr K_(b) = 1.48 xx 10^(-11)`
`:. K_(a) = (10^(-14))/(1.48 xx 10^(-11)) = 6.75 xx 10^(-4)`


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