0.01 M solution of an organic acid is found to have a pH of 4.15. Calculate the concentration of the anion, the ionization constant of the acid and its pK_(a).

0.01 M solution of an organic acid is found to have a pH of 4.15. Calc

1.	0.01 M solution of an organic acid is found to have a pH of 4.15. Calculate the concentration of the anion, the ionization constant of the acid and its pK_(a).
Answer» Solution :PH = 4.15 means `- log [H^(+)] = 4.15 or log [H^(+)] = - 4.15 = bar(5) . 85 or [H^(+)] = 7.08 xx 10^(-5)M` `HA hArr H^(+) + A^(-)` HENCE, at equilibrium `[H^(+)]=[A^(-)] = 7.08 xx 10^(-5)M ~= 7.1 xx 10^(-5)M` `[HA] = (0.01-7.1xx10^(-5))=(0.01-0.000071)M = 0.009929 M` `K_(a) = ([H^(+)][A^(-)])/([HA])=((7.1xx10^(-5))^(2))/(9.929xx10^(-3))=5.08 xx 10^(-7)` `pK_(a) = - log K_(a) = - log (5.08 xx 10^(-7))= 6.29`.