# Martian atmosphere weight and gas constant

## Homework Statement

Determine the mean molecular weight of the Martian atmosphere which is composed of 95% carbon dioxide, 3% nitrogen, 1.6% argon. What is the gas constant for 1kg of such an atmosphere?
Using the value for R from the last problem assuming the surface temperature of Mars is 0° C and has a cubic meter of gas weighs 4.6kg what is the surface pressure?

## The Attempt at a Solution

CO2 = 44 g/mol at 95% = 41.8 g/mol
n2 = 28 g/mol at 3% = 0.84 g/mol
Ar = 40 g/mol at 1.6% = .064 g/mol
Martian atmos molecular weight = 42.704

Gas constant = 1/mean molecular weight?
Gas constant using above formula 0.023417

Surface pressure = pv=mRT
P=(mRT)/V
P=((42.704)(0.023417)(273.15))/1
P=273.14988

Is this even close to right?

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chemisttree
Homework Helper
Gold Member
I've never heard of anyone calculating the gas constant. Its value is independent of composition. Were you taught that the gas constant was inversely proportional to the mean molecular weight of the gas in question?

That's what I have in my notes.

chemisttree
Homework Helper
Gold Member
OK. So I've just learned that there is a universal or "ideal" gas constant and an individual gas constant. I've only ever heard of the ideal gas constant. They are related as:

Ru = R * Mgas

Where
Ru is ideal gas constant
R is individual gas constant or gas mix constant
Mgas is molecular weight of gas or gas mix.

source

So, what units did you use for Mgas?
SI units for the gas constant are J/Kg K. I'm thinking that the gas constant value for a gas mix that is 95% CO2 should be close to the gas constant of CO2 which is ~189 J/Kg K. Yours is ~0.02.

To get my constant I just did 1/42.704 grams which was the 3 gas %'s added up. I may be doing it completely wrong.

Borek
Mentor
OK. So I've just learned that there is a universal or "ideal" gas constant and an individual gas constant.
They use even more strange definitions in climate sciences. I have browsed "Principles of Planetary Climate" once, and on some pages every second concept they introduced used known name, but a twisted definition, or normal definition, but a twisted name. Why do they call molar fraction molar concentration?

Borek
Mentor
To get my constant I just did 1/42.704 grams which was the 3 gas %'s added up. I may be doing it completely wrong.
If you look at the definition chemisttree posted, it can't be 1/something. Solve the formula he posted for the R.