How to find the Density of Air with PV=nRT?

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Discussion Overview

The discussion revolves around finding the density of air using the ideal gas law (PV=nRT) and the implications of air composition on this calculation. Participants explore the relationship between pressure, volume, temperature, and the molar mass of air, particularly in the context of a lab assignment.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests using the ideal gas law but expresses uncertainty about the application, particularly regarding the calculation of moles and density.
  • Another participant emphasizes the need to consider air composition, noting that air is a mixture and lacks a single molar mass, but has an apparent molar mass that can be calculated.
  • There is a suggestion to assume a volume of 1 m³ of gas to calculate the number of moles and mass, which would lead to density calculations.
  • A participant questions whether finding the molar mass from the density of air at STP would aid in determining the molar mass of air.
  • One participant states that the equation d = PM/RT can be used at any temperature and pressure where the ideal gas law is applicable.
  • Another participant argues that it is sufficient to treat air as a mixture of just nitrogen and oxygen, as other gases are present in negligible amounts.

Areas of Agreement / Disagreement

Participants generally agree on the necessity of considering air composition in density calculations, but there is some disagreement regarding the sufficiency of treating air as a mixture of only nitrogen and oxygen. The discussion remains unresolved on the best approach to apply the ideal gas law in this context.

Contextual Notes

Participants express uncertainty about the completeness of the information needed for calculations, particularly regarding the molar mass and the ideal gas law's applicability under different conditions.

lc99
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Homework Statement


Let's say, P = 1.00 atm
and T =273K
and density of air at STP = 1.29 g/L

Homework Equations

The Attempt at a Solution


I'm not too sure...

PV = nRT
n = PV/RT
= P/RT = 1/(0.0821*273) *1.29 g/L ?
 
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PV=nRT alone is not enough, you need air composition as well.

For any gas: assume 1 m3 of gas, can you calculate number of moles? Mass? That will give you density instantly.

For air: air has no molar mass, as it is a mixture, but every mixture has its apparent molar mass (behaves as a gas of such), one that can be calculated as a weighted average of molar masses of the components.

Alternatively: assume 1 m3 of gas. Knowing the air composition, can you calculate volume occupied by the nitrogen? Or its partial pressure? Of other gases?
 
Borek said:
PV=nRT alone is not enough, you need air composition as well.

For any gas: assume 1 m3 of gas, can you calculate number of moles? Mass? That will give you density instantly.

For air: air has no molar mass, as it is a mixture, but every mixture has its apparent molar mass (behaves as a gas of such), one that can be calculated as a weighted average of molar masses of the components.

Alternatively: assume 1 m3 of gas. Knowing the air composition, can you calculate volume occupied by the nitrogen? Or its partial pressure? Of other gases?

im sort of confused because i was told to use the ideal gas law. the question is part of a lab I am doing and I am suppose to find the density of the air based on the lab's temperature and volume.
 
I gave you plenty of hints, have you tried to use them?
 
i
Borek said:
I gave you plenty of hints, have you tried to use them?
figured out how to get the moles of air.. so, the weighted avg of molar mass would be the a lot of gases wouldn't it?

i feel like I am missing some information. would finding the molar mass from the density of air at stp help to find molar mass of air?

so M = dRT/P at stp = 28.913 g/mol

so i can use the equation d = PM/RT for any temperature and pressure?
 
Last edited:
lc99 said:
i

figured out how to get the moles of air.. so, the weighted avg of molar mass would be the a lot of gases wouldn't it?

i feel like I am missing some information. would finding the molar mass from the density of air at stp help to find molar mass of air?

so M = dRT/P at stp = 28.913 g/mol

so i can use the equation d = PM/RT for any temperature and pressure?
You can use this equation at any temperature and pressure at which the ideal gas law is a good approximation for your particular gas.
 
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lc99 said:
the weighted avg of molar mass would be the a lot of gases wouldn't it?

Nope, enough to treat the air as a mixture of just nitrogen and oxygen, other ones are in minute quantities and can be safely ignored.
 
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Borek said:
Nope, enough to treat the air as a mixture of just nitrogen and oxygen, other ones are in minute quantities and can be safely ignored.

Thanks for the help!
 

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