# Homework Help: Efflux times and densities

1. Sep 27, 2008

### Melawrghk

1. The problem statement, all variables and given/known data
This was a lab. We measured efflux times of 3 gases (air, He and CO2) and we have to use that to figure out their densities.
The average efflux time of air was 146.1767s, average efflux time for helium was 137.73 and average efflux time for CO2 was something else, but that's not relevant to the question. The room temperature was 23C, pressure (adjusted) 671.223 mm Hg. Molar mass of air is 28.94kg/kmol

2. Relevant equations
rho = PM/RT <=EQN1
rho1/rho2=((t1)^2)/((t2)^2)) <=EQN2

3. The attempt at a solution
So first I decided to figure out density of air...
rho(air)=89.4891kPa * 28.94kg/kmol /(8.314 [some units] * (23+273.15K)) = 1.0518 kg/m^3

So knowing that, I plugged in the efflux times and rho(air) into EQN2 and got 0.933786 for density of Helium. Then I looked up the actual value of Helium's density and it's supposed to be 0.1787kg/m^3. So the value I got is REALLY off...
Am I doing something wrong? Or was our experiment just horrible with an insane experimental error? Please help...

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