A welder using a tank of volume 7.70×10^−2m^3 fills it with oxygen (with a molar mass of 32.0 ) at a gauge pressure of 3.30×10^5 Pa and temperature of 35.0 C. The tank has a small leak, and in time some of the oxygen leaks out. On a day when the temperature is 23.9 C, the gauge pressure of the oxygen in the tank is 1.80×10^5 Pa . Find the initial mass of the oxygen. i don't understand this crap. i used PV = nRT isolated n = (8.315)(35+273)/ (7.7e-2)(3.3e5) then multiplied it to the molar mass and got the mass. which isnt right. Help? on what im doing wrong? this is the only equation we were given.