What is the equilibrium constant for the weak acid KHP?

Potassium hydrogen phthalate (##"KHP"##) has the ##"C"_8"H"_5"KO"_4## and is known to be a .

When placed in water, ##"KHP"## dissociates completely into the potassium cation ##"K"^(+)## and the hydrogen phthalate anion, ##"HP^(-)##. After the dissociation takes place, ##"HP"^(-)## reacts with water to give the hydronium cation, ##"H"_3^(+)"O"##, and the phthalate anion, ##"P"^(2-)##. So,

##KHP_((aq)) rightleftharpoons K_((aq))^(+) + HP_((aq))^(-)##, followed by

##HP_((aq))^(-) + H_2O_((l)) rightleftharpoons H_3^(+)O_((aq)) + P_((aq))^(2-)##

The expression for the reaction's is

##K_(eq) = ([H_3^(+)O] * [P^(2-)])/([HP^(-)] * [H_2O])##

Because the concentration of water is presumed constant and therefore not included in the expression, the equilibrium constant for this reaction is called acid dissociation constant, ##"K"_"a"##.

##K_a = ([H_3^(+)O] * [P^(2-)])/([HP^(-)])##

Usually, this acid dissociation constant for a particular reaction will be given to you; in ##"KHP"##'s case ##"K"_"a"## is equal to

##K_a = 3.9 * 10^(-6)##, which confirms that ##"KHP"## is a weak acid.

GIven the above expression, you can solve for ##"K"_"a"## by using the equilibrium concentrations of the species involved.