The equilibrium between reactants and products is described by an equilibrium constant. For the balanced reaction:
aA + bB cC + dD
The equilibrium constant, Keq is defined as:
[C]c [D]d Keq = --------- [A]a [B]bwhere the  brackets indicate the concentration of the chemical species.
For the example of water, H2O(l) H+(aq) + OH-(aq)
the equilibrium constant is:
[H+] [OH-] Keq = ---------- [H2O]The concentration of water in a water solution is constant and this expression simplifies to:
Zn (s) + 2 H+(aq) Zn2+(aq) + H2 (g)
PH2 [Zn2+] Keq = ----------- [H+]2where PH2 is the partial pressure of H2, [Zn2+] and [H+] are the molar concentrations of Zn2+ and H+, respectively, and Zn (s) is left out of the Keq expression because it is a pure solid.