What is the expression for Keq for this reaction? 2H2O (g) 2H2 (g) + O2 (g)

For a chemical reaction expressed as,

aA + bB - - > cC + dD

the equilibrium constant is solved by,

Keq = ([C]^c x [D]^d) / ([A]^a x [B]^b)

This means that the product of the concentration of the products raised to exponent that is their numerical coefficients divided by that of the products. This does not include water as it is present in very small amount. Thus, the final expression is,

Keq = [H2]^2 x [O2]^2