The chemical equilibrium

The law of mass action (Guldberg and Waage)

An example

The equilibrium: $N_2O_4$ $\rightleftarrows$ $2NO_2$ obeys the law: $\frac{[NO_2]^2}{[N_2O_4]}$ $=$ $K_c$

The general case

It is proven, that

Law of mass action (Guldberg and Waage) For an equilibrium between species $X$,$Y$,$C$ and $D$ in one phase $aX$ $+$ $bY$ $\rightleftarrows$ $cC$ $+$ $dD$ we always have: $\frac{[C]^c\cdot[D]^d}{[X]^a\cdot[Y]^b}$ $=$ $K_c$ $K_c$ is the equilibrium constant $K_c$ is only a constant at a given temperature

$[C]$, $[D]$, $[X]$ and $[Y]$ are the molarities of the reagents and the products when equilibrium is attained. From that moment on, the molarities don't change no more, provided that there is nothing interfering with the equilibrium.