Finding molar concentration @ equilibrium.

[Jasmine Lam 3F]

On Achieve Week 1 homework #2, it says

At a certain temperature, 0.640 mol SO3 is placed in a 1.50 L container.

2SO3(g)↽−−⇀2SO2(g)+O2(g)

At equilibrium, 0.200 mol O2 is present. Calculate Kc.

The solution shows that it calculates the molar concentration at equilibrium by dividing all the final concentrations by 1.50 after ICE. However, in this example, Dr. Lavelle divides the initial concentration by the given volume, which is before ICE:

1.50 mol PCl5 is placed in a 500 mL container to form PCl3 and Cl2. Kc=1.8. All three substances are gases. What is the composition of the equilibrium mixture?

I'm confused when I'm supposed to divide the concentrations by the volume, is it before ICE or after ICE?

[Chem_Mod]

Both ways will work. However, finding the concentrations BEFORE doing an ICE chart will make the calculations much easier and prevent any possible calculation errors.