Any acid or base can be classified as hard or soft by its apparent preference for the hard and soft reactants. For example, a base B may be classified as hard or soft depending on the equilibrium constant, K eq for the reaction
BH + CH 3 Hg + CH 3 Hg B + + H + .

K eq = × = K s × K a

or log K eq = log K s + log K a

or log K eq = log K s –pK a

K s is the equilibrium constant of the reaction :

CH 3 Hg + + B CH 3 Hg B + K a is the equilibrium constant of the reaction :

BH

+ B + H + For hard base, pK a > log K s , while for soft base, pK a is less than log K s . For the three anions pK a and log K s are as given :

(i)The relative softness of A

– , B – and C – are –