law of mass action
Suppose you run a reaction many times, each time starting with different amounts, and you always wait until it stops changing. Then you compute one particular ratio of the final amounts. The remarkable discovery — made by Guldberg and Waage in the 1860s — is that this ratio comes out to the same number every time, as long as the temperature is the same. The law of mass action is the rule that tells you exactly how to build that ratio.
The law states that for a balanced reaction, at equilibrium the product of the concentrations of the products, each raised to its stoichiometric coefficient, divided by the same product for the reactants, equals a constant — the equilibrium constant. In short, it is the recipe behind K: which substances go on top, which go on the bottom, and what power each is raised to. The exponents are the coefficients from the balanced equation, not anything you measure.
Why it matters: the law of mass action turns the vague idea 'reactions settle into a balance' into a precise, testable equation. The honest caveats are two. First, the coefficients in K come from the overall balanced equation, whereas a rate law's exponents come from the mechanism — the two need not match. Second, strictly the law should use activities rather than raw concentrations, which is why it holds best in dilute or ideal conditions.
For aA + bB ⇌ cC + dD, the law of mass action writes K = ([C]^c [D]^d) / ([A]^a [B]^b). The coefficients a, b, c, d in the balanced equation become the exponents — that single line is the whole recipe for the equilibrium constant.
Products over reactants, each raised to its coefficient.
Do not confuse the exponents in K with the orders in a rate law. K's exponents always come from the balanced overall equation; rate-law orders come from the experimentally determined mechanism and can differ.