chemistry
posted by Kamila .
A container holds 1.00 mol O2, and the pressure in the container is 2.00 atm. A student adds 60.0 g of gas Y to the container. The total pressure (of O2 and gas Y) is 3.00 atm.
What is the molar mass of gas Y?
(V and T constant)

Ok so i also was going to ask this question but I THINK i figured it out.
remember that mols and pressure are proportional. if 1 mol of O2 gives 2 atm of pressure, then how many mols of a gas gives 1 atm of pressure? (because the total is three, so we just need to find out how to get that extra atm of pressure). so i set up a proportion. 1mol/2atm=Xmol/1atm and i got .5 so then i set up 60g Y/.5 mol and i got 120. it makes sense but im not sure if this is how it was supposed to be done. i did get the right answer so i hope i helped a little! 
Yes, 120 is correct.
I worked it with mole fractions.
XO2 = (PO2/Ptotal) = (2/3)
Xunk = (Punk/Ptotal) = (1/3)
Then for oxygen, (1 mol)
(mol O2/(mol O2 + mol unk) = 2/3
Solve for mole unk, then moles = g/molar mass and solve for molar mass.
Your solution is more esoteric than mine.