For 2 SO2(g) + O2(g) equilibrium reaction arrow 2SO3(g), Kp = 3.0 104 at 700. K. In a 2.00-L vessel the equilibrium mixture contains 1.11 g of SO3 and 0.106 g of O2. How many grams of SO2 are in the vessel

Answer 1

You need to work out the concns.
(SO3) = (1.11g/molar mass SO3)/2L = approx 0.007M but you need to do it more accurately as well as calculations that follow. They are estimates also.
(O2) = 1.06/32/2L = approx 0.0015M

Kc = (SO3)^2/(SO2)^2(O2)
Substitute Kc and concns SO3 and O2 and solve for (SO2)

Answer 2

After (SO2) in M, then
mol = M x L
Then mols = grams/molar mass. Solve for grams.