Consider a solution of monoprotic weak acid having dissociation constant `K_(a)`. What is the minimum concentration C in terms of `K_(a)`, such that the concentration of the undissociated acid that the concentration of the undissociated acid can be equated to C within a `10%` limit of error ? Assume that activity coefficient correction are negligible.

Explanation

`HA+H_(2)O hArr H_(3)^(+)O+A^(-)`
`K_(a)= ([H_(3)O^(+)][A^(-)])/([HA])= (C^(2)alpha^(2))/(C(1-alpha))`
within an error of `10% C(1-alpha)`
or `C-Calpha=0.90C`
or `Calpha=0.10C`
`.: K_(a)=((0.10C)^(2))/(0.90C)= (C )/(90)`
or `C= 90 K_(a)`