How many moles of CO(g) will be present when the system reaches equilibrium?

Question

The following reaction has an equilibrium constant of 2.75 x 10-7 at a given temperature.

HCHO(g) CO(g) + H2(g)

If you have 0.75 moles of HCHO(g) in a 1.00L container, how many moles of CO(g) will be present when the system reaches equilibrium?

Answer 1

Set up an ICE table

HCHO(g) ==> CO(g) + H₂(g)

0.75................0...........0........Initial

-x.................+x...........+x.......Change

0.75-x............x.............x.......Equilibrium

K = 2.75x10^-7 = [CO][H₂] / [HCHO]

2.75x10^-7 = (x)(x) / 0.75-x (assume x is small relative to 0.75 and ignore it to avoid using the quadratic)

2.75x10^-7 = x² / 0.75

x² = 2.06x10^-7

x = 4.5x10^-4 (note: this is much less than 0.75 so the above assumption was valid)

So, when the system reaches equilibrium there will be 4.5x10^-4 mols of CO