Martian atmosphere weight and gas constant

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Discussion Overview

The discussion revolves around calculating the mean molecular weight and gas constant of the Martian atmosphere, which is primarily composed of carbon dioxide, nitrogen, and argon. Participants explore the implications of these calculations for understanding the atmospheric pressure on Mars, with a focus on the theoretical and practical aspects of gas constants in different contexts.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant calculates the mean molecular weight of the Martian atmosphere and proposes a method for determining the gas constant and surface pressure, but questions the accuracy of their results.
  • Another participant asserts that the gas constant is independent of the gas composition and questions the initial assumption that it is inversely proportional to the mean molecular weight.
  • A participant clarifies the distinction between the universal gas constant and the individual gas constant, providing a formula that relates them, and suggests that the calculated gas constant should be close to that of carbon dioxide.
  • There is confusion regarding the units used for the molecular weight and the calculation of the gas constant, with one participant expressing uncertainty about their approach.
  • A later reply raises concerns about terminology used in climate sciences, indicating that definitions may vary and lead to misunderstandings.

Areas of Agreement / Disagreement

Participants express differing views on the calculation of the gas constant and its relationship to molecular weight. There is no consensus on the correct approach or the validity of the calculations presented.

Contextual Notes

Participants mention various definitions and units related to gas constants, indicating potential limitations in understanding and applying these concepts correctly. There is also a noted confusion regarding the terminology used in different scientific contexts.

jg24
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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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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.
 
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.
 
chemisttree said:
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?
 
jg24 said:
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.
 

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