equilibrium constant and mol

[cootcoot]

for           N2(g)+ 3H2(g) ----> 2NH3(g)    in 1.0 L vessel

it pretty much asks if in the mixture there is .2 mol N2 .2 mol H2 .2 mol NH3, find equ. constant.


the question i have is:  do i use the .2 mol for each to then find the concentration ... eg k=(.2)^2/ (.2)x(.2)^3

or do i go, well, theres .2 mol H2 therfore there is .06666 mol N2 and .13333 mol NH3?

[Collin Li]

The definition of the equilibrium constant is:

The value of the ratio between the concentration of the products and the concentration of the reactants at equilibrium.

So it is necessary that the question says that at equilibrium, the mixture has so and so... because I could just find any ratio of the concentration of products and reactants, and it wouldn't necessarily be the equilibrium constant. The special thing about the number is that it is a ratio of the concentrations at equilibrium.

I will assume that the question did say that: so since we have 0.2 mol of , 0.2 mol of , and 0.2 mol of , and we have a volume of 1.0 L:





Using:



Therefore:

(don't forget the units! -- which happen to be meaningless, by the way)

[Collin Li]

or do i go, well, theres .2 mol H2 therfore there is .06666 mol N2 and .13333 mol NH3?

This assumes the reaction goes to completion, and I see you are using calculations for limiting reagents here. That's not the right approach because you don't know how far the reaction will actually go (not fully, assuming the K is not infinite).