An oxygen sample has a volume of 4 L at 27C and 900 torr. How many oxygen molecules does it contain

An oxygen sample has a volume of 4 L at 27C and 900 torr. How many oxygen molecules does it contain?
So I'm pretty sure I have to use PV=nRT, but I keep getting it wrong and I can't seem to catch my mistake. Some help would be greatly appreciated!

Yes use PV=nRT to find the moles first, but remember to convert torr to atm's and Celsius to Kelvin.

PV=nRT

(1.20 atm) (4.0 L) = n (.0821) (300 K)

n = (1.20 atm) (4.0 L) / (.0821) (300 K)

n = .20 moles

Now convert moles to molecules.

.20 moles O2 x [6.022 x 10^23 molecules / 1 mole] = 1.20 x 10^23 molecules of O2

Hope I helped!