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

[Kitaek Lee]

Homework Statement

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.

Homework 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.

The Attempt at a Solution

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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?

 

[Bystander]

Gauge pressure.

 

[Kitaek Lee]

Gauge pressure.

thank you for replying, but can you please explain more specific, please?

 

[Bystander]

What is the definition of "gauge pressure?"

 

[gneill]

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

 

[rude man]

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