Answer: [tex]K=\frac{[H_3PO_4]}{[H^+]^3[PO_4^{3-}]}[/tex]
Explanation:
Equilibrium constant is the ratio of product of concentration of products to the product of concentration of reactants each term raise to their stochiometric coefficients.
Thus for the reaction:
[tex]3H^+(aq)+PO_4^{3-}(aq)\rightarrow H_3PO_4(aq)[/tex]
[tex]K=\frac{[H_3PO_4]}{[H^+]^3[PO_4^{3-}]}[/tex]