# Oxygen (O2) is about 20% of our air, with about 80% nitrogen and a bunch of minor components. In Colorado Springs, where I grew up, the elevation is ~ 6500 feet and atmospheric pressure is only around 9 psi (vs. 14.7 psi at sea level). Under those conditions and assuming T = 300 K, calculate the number of moles of oxygen per liter, the number of oxygen molecules per liter, and the density of oxygen in g/L.

A)Calculate the number of moles of oxygen per liter

My work:
n=PV/RT =
(0.612 atm x 0.2 L)/((0.08206 atm x L)/(mol x K)) x 300 K = 0.00497 mol O2

0.00497 mol O2/0.2 L = 0.0249

Is it correct to divide by 0.2 from 20% of O2?

B) the number of oxygen molecules per liter

0.00497 mol O2 x (6.02 x 10^23 molecules/1 mol O2)
= 2.99 x 10^21 O2 molecules

(2.99 x 10^21)/0.2 L = 1.50 x 10^22

Is it correct to divide by 0.2 L?

My answer: 1.50 x 10^22 O2 molecules per liter

C) Density of oxygen

d= PM/RT

d= (0.612 atm x 32.00 g/mol)/(0.08206 x 300) = 0.796 g/L

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1. I worked c the same way you did and another way and got the same answer each way. Then I used both ways to to do A and came up with your answer. I assume the others are ok. Here is the OTHER way to do density.
Density at STP is 32/22.4 = about 1.43 g/L. Not convert from STP to the new conditions.
1.43 g/L x (9/14.7) x (273/300) = ?

Then convert to mols O2 in a liter. 0.796/32 = 0.0249

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