Chemistry
posted by Margaret .
At a particular temperature, 13.7 mol of SO3 is placed into a 3.9L rigid container, and the SO3 dissociates by the reaction given below.
2 SO3(g) 2 SO2(g) + O2(g)
At equilibrium, 3.8 mol of SO2 is present. Calculate K for this reaction.

13.7mol/3.9L = about 3.5
3.8/3.9 = about0.97
You need to do these more accurately.
........SO3 ==> 2SO2 + O2
I.......3.5.......0......0
C.......2x.....2x.......x
E......3.52x...2x........x
The problem tells you that at equilibrium (SO2) = 2x = 3.8/3.9 = 0.974M
Knowing x, you can calculate SO2m O2, and SO3, then substitute into the Kc expression and solve for Kc. 
it says my answer is still wrong. I got 0.017683 but it says the right answer is 0.071777081878092.