the equilibrium concentrations were found to be [H2O]=0.250 M , [H2]=0.600 M , and [O2]=0.800 M . What is the equilibrium constant for this reaction?

Keq = 0.217

Explanation:

Let's determine the equilibrium reaction.

In gaseous state, water vapor can be decomposed to hydrogen and oxygen and this is a reversible reaction.

2H₂(g)  + O₂(g)  ⇄  2H₂O (g)         Keq

Let's make the expression for the equilibrium constant

Products / Reactants

We elevate the concentrations, to the stoichiometry coefficients.

Keq = [H₂O]² / [O₂] . [H₂]²

Keq = 0.250² / 0.8 . 0.6² =  0.217

elements make up everything, and they combine to make compounds.

the answer is c