Answer
$[NO] = 0.50 atm$
$[NO_2] = 0.02 atm$
Work Step by Step
1. Write the equilibrium constant expression:
- The exponent of each concentration is equal to its balance coefficient.
$$K_c = \frac{[Products]}{[Reactants]} = \frac{[ NO ]^{ 2 }[ O_2 ]}{[ NO_2 ]^{ 2 }}$$
2. At equilibrium, these are the concentrations of each compound:
$[ NO_2 ] = [NO_2]_0- 2x$
$[ NO ] = 2x$
$[ O_2 ] = x$
$$158 = \frac{(2x)^2(x)}{([NO_2]_0 -2x)^2}$$
3. If $[O_2] = 0.25 \space atm$, then x = 0.25
$$158 = \frac{(2(0.25))^2(0.25)}{([NO_2]_0 -2(0.25))^2}$$
$[NO_2]_0 = 0.52 M$
$[NO_2] = 0.52 - 0.25*2 = 0.02 atm$
$[NO] = 2(0.25) = 0.50 atm$
