A weak monobasic acid is found to be 4% ionided at 0.1 M concentration. Calculate the value of the ionisation constant.

Explanation

Let the weak monobasic acid be H.A . Its ionisation in a aqueous solution can be represented as :
`{:(,HA(aq),hArr,H^(+)(aq),+,A^(-)(aq)),("Initial molar concentration",C,,0,,0),("Equilibrium molar concentration",C(1-alpha),,alpha,,alpha):}`
`Here " "C=0.1 M,alpha =4% 4/(100) =4 xx 10^(-2)`
`"At equilibrium, " " [HA]=C(1-alpha)=0.1 Mxx (1-4xx 10^(-2)) =9.6 xx 10^(-2) M`
`[H^(+)]=[A^(-)]=Calpha=0.1Mxx4xx10^(-2)=4xx10^(-3) M`
`K_(a) =[[H^(+)][A^(-)]]/[[HA]]=((4xx10^(-3))xx(4xx10^(-3)))/(9.6xx10^(-2))=1.67 xx 10^(-4)`