# Basic ideal gas PV=nRT question ( need help fast )

1. Sep 27, 2009

### 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 isnt right. Help? on what im doing wrong? this is the only equation we were given.

2. Sep 27, 2009

### 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)

3. Sep 27, 2009

### Luongo

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

4. Sep 27, 2009

### 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?

5. Sep 27, 2009

### Luongo

nope but i get it now, thanks.