For the general equation of the nature xP + yQ ⇌ mR + nS, the expression for the equilibrium constant is

Analyze the question: You are asked for the equilibrium constant expression (Kc) for the general reversible reaction xP + yQ ⇌ mR + nS.

Concept (Law of Mass Action): At equilibrium, the equilibrium constant is the ratio of the product of the concentrations of the products to the product of the concentrations of the reactants. Each concentration is raised to the power of its coefficient in the balanced equation.

Solution:

1. Products are R and S, with coefficients m and n. The numerator is [R]^m [S]^n.
2. Reactants are P and Q, with coefficients x and y. The denominator is [P]^x [Q]^y.
3. Putting them together:

Kc = [R]^m [S]^n / ([P]^x [Q]^y)

Checking the options:

• A: This has only reactants and a stray constant k. It is wrong.
• B: The coefficients are used as multipliers instead of powers. It is wrong.
• C: This is the inverse, reactants over products. It is wrong.
• D: Products over reactants, with coefficients as powers.

Answer: D

Exam tip: Remember "Products over Reactants, coefficients become powers." Also, pure solids and liquids are left out of the expression, which JAMB often tests in follow-up questions.