It has been found that the pH of a 0.01M solution of an organic acid is 4.15. Calculate the concentration of the anion, the ionization constant of the acid and its `pK_(a)`.

Explanation

Suppose the organic acid is HA.
` " " HA hArr H^(+) +A^(-)`
` " " pH =- log _(10) [H^(+) ]`
` " " 4.15 = - log_(10) [H^(+) ]`
` therefore [H^(+) ]=" antilog"_(10) (-4.15 ) = 7.08 xx 10 ^(-5) M.`
` " " [A^(-)]=[H^(+) ] = 7.08 xx 10 ^(-5) ` M.
` therefore " " K_a = ({H^(+) ][A^(-)])/([HA]) `
`= (7.08 xx 10 ^(-5) xx 7.08xx 10 ^(-5))/(0.01) = 5.01 xx 10^(-7)`
` " " (because ` dissociations of the acid is very low)
` rhoK_a = - log_(10) K_a= - log_(10) (5.01xx 10^(-7)) = 6.3002`