Basic ideal gas PV=nRT question ( fast )

[Luongo]

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 isn't right. Help? on what I am doing wrong? this is the only equation we were given.

 

[jamesrc]

- you are given gauge pressures, but P in your ideal gas law equation is in absolute pressure - you need to convert gauge pressure to absolute pressure (that is, you need to add atmospheric pressure)

- check your algebra when calculating n - looks upside down to me...

- otherwise there's nothing wrong with the way you are going about it (use the ideal gas law to find the amount of oxygen in the tank [n] and use the molar mass to find the corresponding mass)

 

[Luongo]

- you are given gauge pressures, but P in your ideal gas law equation is in absolute pressure - you need to convert gauge pressure to absolute pressure (that is, you need to add atmospheric pressure)

- check your algebra when calculating n - looks upside down to me...

- otherwise there's nothing wrong with the way you are going about it (use the ideal gas law to find the amount of oxygen in the tank [n] and use the molar mass to find the corresponding mass)

sorry typo, yes you're right i have to add 1atm of pressure but why? we were never told the reasoning

 

[jamesrc]

Well, it's more or less the same reason you had to add 273 to your temperature - the ideal gas law is defined in terms of absolute pressure and temperature.

Did you ever go over the difference between gauge pressure and absolute pressure in class?

 

[Luongo]

Well, it's more or less the same reason you had to add 273 to your temperature - the ideal gas law is defined in terms of absolute pressure and temperature.

Did you ever go over the difference between gauge pressure and absolute pressure in class?

nope but i get it now, thanks.