Carbon monoxide and chlorine gas react to form phosgene:

CO(g)+Cl2(g)⇌COCl2(g) Kp = 3.10 at 700 K
If a reaction mixture initially contains 319torr of CO and 220torr of Cl2, what is the mole fraction of COCl2 when equilibrium is reached?

1 answer

.......CO + Cl2 ==> COCl2
I.....319...220......0
C......-x....-x......x
E...319-x...220-x....x

Substitute the E line into the Kp expression and solve for x, 319-x, and 220-x. Then add mols of each to find total mols.
XCO = nCO/total mols
XCl2 = nCl2/total mols
XCOCl2 = nCOCl2/total mols.
The problem only asked for XCOCl2 but all of them can be calculated easily after you have the first one.