The `pH` of `0.1M` monobasic acid is `4.50`. Calculate the concentration of species, `H^(o+), A^(Θ)`, and `HA` at equilibrium. Also determine the value of `K_(a)` and `pK_(a)` of the monobasic acid.

Explanation

`pH = - "log" [H^(+)]`
Therefore, `[H^(+)] = 10^(-pH) = 10^(-4.50)`
`= 3.16 xx 10^(-5)`
`[H^(+)]= [A^(-)] = 3.16 xx 10^(-5)`
Thus, `K_(a) = [H^(+)][A^(-)] //[HA]`
`[HA]_("equlbm") = 0.1 - (3.16 xx 10^(-5))^(2)// 0.1 = 1.0 xx 10^(-8)`
`K_(a) = (3.16 xx 10^(-5))^(2)//0.1=1.0xx10^(-8)`
`pK_(a) = -"log"(10^(-8)) = 8`
Alternatively, "Percent dissociation" is another useful method for measure of strength of a weak acid and is given as :
Percent dissociation
`= [HA]_("dissociated")//[HA]_("initial")xx 100% , (7.32)`