Equilibrium Constant Kc (A + B ⇌ C + D)

Cato Guldberg and Peter Waage, two brothers-in-law working in Kristiania (now Oslo), published the law of mass action in 1864: at equilibrium the products of the concentrations, each raised to its stoichiometric coefficient, sit in a fixed ratio. For the common one-to-one case A + B ⇌ C + D that ratio is just [C][D]/([A][B]). Their paper appeared in Norwegian and was ignored for a decade until a French translation reached van 't Hoff and Ostwald.

The identical expression evaluated with any set of concentrations, not necessarily equilibrium ones, is the reaction quotient Q, and comparing Q with Kc predicts which way the reaction will run: Q < Kc means it goes forward, Q > Kc means it reverses, Q = Kc means it is already there. Worked example: with [C] = [D] = 0.60 M and [A] = 0.20 M, [B] = 0.30 M, the quotient is 0.36/0.060 = 6.0, so if Kc were 2.0 the mixture would run backwards to consume product. Note that this calculator assumes all four coefficients are 1; with other coefficients each concentration needs its own exponent.