Finding the mass of a quantity of gas given P, V, T

1. Jan 10, 2016

Kitaek Lee

1. The problem statement, all variables and given/known data

A welder using a tank of volume 8.00×10−2 m^3 fills it with oxygen (with a molar mass of 32.0 g/mol ) at a gauge pressure of 3.10×105 Pa and temperature of 38.9 ∘C. The tank has a small leak, and in time some of the oxygen leaks out. On a day when the temperature is 23.0 ∘C, the gauge pressure of the oxygen in the tank is 1.75×105 Pa .

Find the initial mass of oxygen.

2. Relevant equations

pV=nRT
where p is pressure, v is volume, n is number of mole, R is 8.31442(since p is Pa and V is in m^3), and T is kelvin.
m total = n*M ; where n is number of moles and M is molar mass.
3. The attempt at a solution

pV = nRT

= 3.10 * 10^5 * 8.00 * 10^(-2) = n*8.314472* 312.05

n = 9.558

9.558 * 0.032 kg/mol = 0.305856

however, 0.305856 is incorrect. where did i make mistake?

2. Jan 10, 2016

Bystander

Gauge pressure.

3. Jan 10, 2016

Kitaek Lee

4. Jan 10, 2016

Bystander

What is the definition of "gauge pressure?"

5. Jan 11, 2016

Staff: Mentor

Mentor's note: Thread title has been changed to make it descriptive of the problem.

6. Jan 12, 2016

rude man

Did you take cognizance of the fact that n has changed?