In atmosphere, `SO_(2)` and `NO` are oxidised to `SO_(3)` and `NO_(2)`, respectively,w hcih react with water to given `H_(2)SO_(4)` and `HNO_(3)`. The resultant solution is called acid rain. `SO_(2)`dissolves in water to form diprotic acid.
`SO_(2)(g) +H_(2)O(l) hArr HSO_(3)^(Theta) + H^(o+), K_(a_(1)) = 10^(-2)`.
`HSO_(3)^(Theta) hArr SO_(3)^(2-) + H^(o+), K_(a_(2)) = 10^(-7)`
and for equilibrium,
`SO_(2)(aq) + H_(2)O (l) hArr SO_(3)^(2-)(aq) +2H^(o+)(aq)`
`K_(a) = K_(a_(1)) xx K_(a_(2)) = 10^(-9) at 300K`.
The `pH` of `0.01M` aqueous solutioon of sodium sulphite `(Na_(2)SO_(3))`

Options

A. `4.5`
B. `8.5`
C. `9.0`
D. `9.5`

Answer

D

Explanation

`Na_(2)SO_(3)` is a salt of `W_(A)//S_(B)`.
`pK_(a_(2)) =- log (10^(-7)) = 7`.
(use `pK_(a_(2))` value since `SO_(3)^(2-)` is used in salt formation)
`:. pH = (1)/(2) (pK_(w) + pK_(a_(2)) + logC)`
`=(1)/(2) (14 +7 + log 0.01) = (1)/(2) (21 - 2) = 9.5`