The dissociation constant of a weak acid `HA si 4.9 xx 10^(-8)`. After making the necessary approximations, calculate
i. Percentage ionisation
ii. `pH`
iii. `overset(Theta)OH` concentration in a decimolar solution of the acid. Water has a `pH of 7`.

Explanation

`overset(0.1M)(HA) Leftrightarrow overset(0)(H^+) +overset(0)(A^-)` Initial concentration
`0.1 (1-x) " 0.1x 0.1x"` Concentration at eqb.
(i) `K_a =([H^+] [A^-])/([HA])=(0.1 xx 0.1x)/(0.1 (1-x))=4.9 xx 10^(-8)`
`x=7 xx 10^(-4)`
percentage ionisation `=7 xx 10^(-2)%`.
(ii) `[H^+]=0.1 xx x=0.1 xx 7 xx 10^(-4)=7 xx 10^(-5) "mole/litre".`
`pH =-log (7 xx 10^(-5))=4.15`
(iii) We know,
`K_(w) -[H^+] [OH^(-)]`
`1 xx 10^(-14) =(7 xx 10^(-5)) [OH^(-)]`
`[OH^(-)]=1.43 xx 10^(-10) "mole/litre".`