# Calculating Density of Oxygen at STP

tomrh3
Ok, I know I have to use the ideal gas law, but am not exactly sure how to apply that to finding the density.

PV=nRT (ideal gas law)

1atm (volume) = n(number of moles - can I just use 1 for ease?)(8.315 J(mol•K) (temperature)

Not quite sure where to go from here, any help would be appreciated just to get me in the right direction. Do I need the density of Oxygen at another temperature in order to do this?

SteveDB
Tom,
density of an element at stp is its mass/volume.
Since-breathable- oxygen is 2 atoms at standard temp(293-298K) and pressure(1atm), you need to account for O2-- 16 grams per mole, i.e., n=2.
Do you have the "unit" volume?
nrt/p = V.
you can get its atomic mass from the periodic chart.
A decent (one bought from the college bookstore for \$5.00) chart would also have the unit density on it as well.

tomrh3
SteveDB said:
Tom,
Since-breathable- oxygen is 2 atoms at standard temp(293-298K) and pressure(1atm), you need to account for O2-- 16 grams per mole, i.e., n=2.
Do you have the "unit" volume?
nrt/p = V.

Shortly after my post I realized that the atomic mass was the piece that I was missing, so to find n I did n=(mass)/(molecular mass), n=1gram/16, and after plugging that all in, I got a final density of 1.40 (which is basically much that of oxygen). Was I right to use n=1/16 or was it n=2? Thanks for the help.

Homework Helper
The molar volume at stp is 22.4141L as I recall. This is easily calculated with the ideal gas law using 1 mole and stp, be sure to use correct units for your R. From there the density will be molar mass/molar volume.

pacoranal
I found this claculation by chance and I noticed a mistake. Molecular mass for O2 is 32, not 16 because the O2 molecule is formed by two O atoms which mass is 16 each.