The pH of 1L of a buffer obtained by the combination of a weak acid (HA) and its salt (NaA) is 5.0. What will be the ionisation constant of HA if the concentrations of HA and NaA in the buffer are 0.3 (M) and 0.4 (M) respectively?

Explanation

`pH=pK_a+log([NaA])/([HA])=-log(4xx10^(-4))+log(0.15)/(0.075)`
=3.7
`:.[H_3O^+]=10^(-pH)=10^(-3.7)(M)=2xx10^(-4)(M)`