A homogeneous equilibrium has everything present in the same phase. The usual examples include reactions where everything is a gas, or everything is present in the same solution.
In writing an expression for equilibrium constant (Keq), we can start at a general case with the equation:
Where A,B,C and D are the chemical species involved and a.b.c and d are the coefficients in the balanced chemical equation. According to the law of mass action, the equilibrium condition is described by the expression:
If you allow this reaction to reach equilibrium and then measure the equilibrium concentrations of everything, you can combine these concentrations into an expression known as equilibrium constant.