For 2 SO2(g) + O2(g) equilibrium reaction arrow 2 SO3(g), Kp = 3.0 104 at 700 K. In a 2.00-L vessel the equilibrium mixture contains 1.21 g of SO3 and 0.183 g of O2. How many grams of SO2 are in the vessel?

Answer 1

mols SO3 = grams/molar mass = ?
Then M SO3 = mols/L. Let's call this about 0.0075 M but that's just an estimate. You need to do it much more accurately.

mols O2 = grams/mola mass = ?
Then M O2 = mols/L. Let's call this about 0.003 M. Again, you should do it more accurately.

.......2SO2 + O2 ==> 2SO3
E........x..0.003.....0.0075

Substitute the E line into Kp expression and solve for x = (SO2)
Convert that from M to mols in 2 L to grams in 2 L.