Chapter 10 - Reaction Rates and Chemical Equilibrium - 10.3 Equilibrium Constants - Questions and Problems - Page 383: 10.17

The numerical value for $K_c$ for this reaction is 1.5.

1. The formula for the equilibrium constant expression is: $$K_c = \frac{[Products]}{[Reactants]}$$ Also, the coefficients in the equation are the exponents of the concentrations. $$K_c = \frac{[NO_2]^2}{[N_2O_4]}$$ 2. Substitute the concentrations into the expression and evaluate it: $$K_c = \frac{(0.21)^2}{(0.030)} = 1.5$$