From a problem set...

Problem Set #4, Question 2:

2. Carbon dioxide reacts with ammonia (NH₃) in the gas phase to produce formamide (HCONH₂) and oxygen. If 15.215g of CO₂(g) and 4.139g of ammonia are combined in a 3.50L vessel and allowed to react equilibrium, the formamide concentration is found to be 24.8mM. What is the equilibrium constant for this reaction? Is this reaction product-favored or reactant-favored?

Balanced equation:
2 CO₂(g) + 2 NH₃(g) <--> 2 HCONH₂(g) + O₂(g)

[CO₂]_initial = 15.215g / 44.009g/mol / 3.50L = 0.098778M
[NH₃]_initial = 4.139g / 17.031g/mol / 3.50L = 0.069436M
[HCONH₂]_initial = 0M
[O₂]_initial = 0M
{NOTE: Once again, I'm carrying too many sig figs here on purpose because this is the middle of the calculation. I'll round later.}

Set these up in a table, I don't have tables active in the blog so....

Change in:
[CO₂] = -2x [NH₃] = -2x [HCONH₂] = +2x [O₂] = +x

So at equilibrium,
[CO₂]_eq = (0.098778-2x)M
[NH₃]_eq = (0.069436-2x)M
[HCONH₂]_eq = (0+2x)M
[O₂]_eq = (0+x)M

The problem states that the equilibrium concentration of formamide is 24.8mM, so we can use that number to find the value of "x".
[HCONH₂]_eq = (0+2x)M = 24.8mM
x = 12.4mM = 0.0124M

Plugging in to find all equilibrium concentrations:
[CO₂]_eq = (0.098778-2x)M = (0.098778-0.0248)M = 0.07398M
[NH₃]_eq = (0.069436-2x)M = (0.069436-0.0248)M = 0.04464M
[HCONH₂]_eq = (0+2x)M = 0.0248M
[O₂]_eq = (0+x)M = 0.0124M

K_c = { (0.0248)²(0.0124) } / { (0.07398)²(0.04464)² } = 0.699
Slightly reactant-favored.