Which of the following is the correct equilibrium constant expression for the reaction: 3H+(aq) + PO4^3-(aq) ⇌ H3PO4(aq)?

• A. K = [H+]^3[PO4^3-] / [H3PO4]
• B. K = [H3PO4] / ([H+]^3[PO4^3-])
• C. K = [H3PO4] / [H+][PO4^3-]
• D. K = [H+]^3 / ([H3PO4][PO4^3-])

Answer: (B)

Verified step by step guidance

1. Identify the general form of the equilibrium constant expression for a reaction: For a reaction aA + bB ⇌ cC + dD, the equilibrium constant expression is K = ([C]^c[D]^d) / ([A]^a[B]^b).
2. Apply the general form to the given reaction: 3H+(aq) + PO4^3-(aq) ⇌ H3PO4(aq). Here, the reactants are H+ and PO4^3-, and the product is H3PO4.
3. Write the equilibrium constant expression: The expression will be K = [H3PO4] / ([H+]^3[PO4^3-]), where [H3PO4] is the concentration of the product, and [H+]^3 and [PO4^3-] are the concentrations of the reactants raised to the power of their stoichiometric coefficients.
4. Compare the given options with the derived expression: The correct expression is K = [H3PO4] / ([H+]^3[PO4^3-]), which matches one of the provided options.
5. Understand why the other options are incorrect: The other options either do not correctly represent the stoichiometric coefficients or incorrectly place reactants/products in the numerator/denominator of the expression.