# Mass of oxygen

1. Jan 21, 2010

### Zhalfirin88

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

What is the total mass (in grams) of oxygen in a room measuring 4.0 by 7.0 by 2.5? Assume that the gas is at STP and that air contains 20.95% oxygen by volume.

3. The attempt at a solution
I don't know where to begin.

2. Jan 21, 2010

### mgb_phys

3. Jan 21, 2010

### Zhalfirin88

Yes.

4. Jan 21, 2010

### mg0stisha

Then you have everything you need except the variable n, or the number of moles. There's also an equation that relates n (number of moles) to the mass. Do you know of this equation?

5. Jan 21, 2010

### mgb_phys

Then that gives you the number of moles of air -
Find the average molecular mass of air by assuming that the rest is N2 (mass=28)

Then you know the number of moles of O2 - and so the mass

6. Jan 21, 2010

### Zhalfirin88

V = (4*7*2.5) = 70m^3
P = 1atm
T = 273 K
R = .000082
n = PV/RT
n = 3126.95
1 mole of oxygen is 32 g/mol so
32 * 3126.95 = 1000062.53 but you only want 20.95% of that so
1000062.53 * .2095 = 20963.10
= 2.10 x 10^ 4

7. Jan 22, 2010

### Staff: Mentor

Pounds? Talents? Stones? Carats? Maunds? Catties? Wiarduneks?

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