# Oxygen pressure

1. Jun 26, 2011

### Tigrisje

In a spherical vessel with a hydrogen H2 and oxygen mixture O2 it is stated in a problem that we can derive that the concentration of oxygen [O2] can be derived to be:

P / 3RT. With P the total pressure, R the gas constant en T temperature?

How on earth is this possible? I thought the ideal gas law stated that p = n R T (with n the concentration per unit volume). Any help?

2. Jun 27, 2011

### chemisttree

It would depend on how you define 'n'. It is usually defined as total moles of gas. In this case it is a mixture of H2 and O2. If we let n = moles of oxygen AND we assume that the mixture is stoichiometric (ie. 2H2 + O2) you get the result you posted.

See if you can rearrange the general equation, n = PV/RT into a concentration (moles per liter), concentration = P/3RT. If the general gas law only defines the system for all gases, where does that '3' come from?