# Homework Help: Calculate Molecular Mass

1. Jan 24, 2012

### annalise17

1. The problem statement, all variables and given/known data

I need to calculate the mas of a molecule of an unknown gas in the atmosphere from the values:

T = 500 K
mass of the planet = 1.5 × 1024 kg
radius is 3.0 × 106 m
universal constant of gravitation, G = 6.67 × 10^−11 N m^2 kg^−2
P at ground level = 2.0 × 10^7 Pa
P at 50km altitude = 2.0 × 10^5 Pa

2. Relevant equations

g=kT/mλ

but I can't find an equation involving G :(

3. The attempt at a solution

I am completely stuck, if I can get ahint where to start I'll have a go and see if I get anywhere.

2. Jan 24, 2012

### Spinnor

On Earth

F = mg = GmM/R^2 so g = GM/R^2

3. Jan 25, 2012

### annalise17

Right, I've got so far as calculating g now and have calculated a very low value for g compared to earth which is consistent with having a smaller mass than earth. I've also worked out an equation for m by rearranging λ=kT/mg ut the value doesn't match that of the two gases it could be by a factor of 10^3.

Last edited: Jan 25, 2012