Equilibrium is Organization!!

The key to equilibrium problems is almost always figuring out the best way to organize the data presented in the problem or experiment.  This is most often done by putting together a table.  Consider the following problem.

0.80mols NF₃(g) and 0.60mols O₂(g) are combined in a 1.00L vessel and allowed to reach equilibrium. At equilibrium, the concentration of NF₃(g) is found to be 0.30M. What is the value of the equilibrium constant for this reaction?
Write a balanced chemical equation:

2 NF₃(g) + O₂(g) ↔ 2 NO(g) + 3 F₂(g)

Set up a table to organize the data from the problem:

	2 NF3(g) +	O2(g) ↔	2 NO(g) +	3 F2(g)
[ ]initial	0.80 M	0.60 M	0 M	0 M
Δ[ ]	- 2x	- x	+ 2x M	+ 3x M
[ ]equil	(0.80-2x) M	(0.60-x) M	2x M	3x M

Write an expression for the equilibrium constant:

Plug in values from the table:

Determine "x" from the problem.  We are told that [NF3]eq=0.30M, and we see in the table that [NF3]eq=(0.80-2x), so:

0.30 = 0.80-2x

x = 0.25

Plugging in:

K = 3.4

This equilibrium is (very slightly) product-favored.

Practice, practice, practice...