# Mole Fraction of O2 to Mass of O2

1. Jan 21, 2012

### sirchicken

Hi Everyone,

I am trying to calculate the mass of oxygen and keep running into dead ends and was wondering if anyone could offer any insight.

I am trying to find the resulting mass of oxygen in a 6m x 6m x 6m enclosure at standard temperature and pressure when the mole fraction of oxygen is reduced from 0.21 to 0.158. Can anybody offer any advice?

Thanks!

2. Jan 21, 2012

### Redbelly98

Staff Emeritus
What is your specific application, i.e. what is it you are trying to accomplish?

Anyway, I would start by calculating the mass as if the mole fraction were 100%, and use the ideal gas law to calculate how many moles of oxygen would be in the container.

p.s. welcome to PF.

3. Jan 22, 2012

### sirchicken

I am trying to find out what the mass of the oxygen in the enclosure will be if the mole fraction of oxygen from 0.21 to 0.158.

I am assuming that at stp the mole fraction of oxygen is equal to 0.21 and the mole fraction of nitrogen is 0.79. Then a fire is introduced and reduces the mole fraction of oxygen to 75% of its ambient value. I am trying to calculate the resulting mass of oxygen in the enclosure so that I can determine how long the fire can burn.

Thanks!

4. Jan 22, 2012

### Redbelly98

Staff Emeritus
Okay. Not sure why you want to test this at 32 °F / 0 °C (that's what standard temperature is), rather than at room temperature, but you know more about what you are trying to do than I.

At standard temperature and pressure, 1 mole of an ideal gas occupies 22.4 liters.
You can use google to figure out how many 22.4-liter volumes would be contained in a 6x6x6 m^3 space; just go to google and enter
(6*6*6 m^3) / (22.4 liters)​
That will tell you how many moles of gas molecules (of all kinds) are in the 6x6x6 m3 space.

Multiply the number of moles of gas by the fraction that is oxygen, i.e. 0.21 or 0.158, to get the number of moles of oxygen.

Multiply the number of moles of oxygen by the grams-per-mole for oxygen -- that would be 32 -- and you'll have the mass in grams of oxygen that is in the container.