Can we use molecular speed of gas formula also for the air?

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Discussion Overview

The discussion centers on the applicability of the molecular speed of gas formula, $$v=\sqrt{\frac{3RT}{M}}$$, specifically in the context of air, which is a mixture of different gases. Participants explore whether this formula can be used for air and how it relates to the speed of sound.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • Some participants propose that the molecular speed formula can be applied to air, despite it being a mixture of gases.
  • One participant calculates the molecular weight of air as approximately 28.96 g/mole, based on the composition of nitrogen, oxygen, and argon.
  • Another participant suggests that modeling air as a diatomic ideal gas can yield useful results, given its primary components are diatomic gases.
  • It is noted that the computed RMS velocity is believed to relate to the speed of sound, with a proportional relationship in ideal gases.
  • A specific relationship between the speed of sound and the RMS molecular speed is mentioned, involving the ratio of specific heats for diatomic gases.

Areas of Agreement / Disagreement

Participants generally agree that the molecular speed formula can be applied to air, but there is some uncertainty regarding the implications of this application, particularly in relation to the speed of sound.

Contextual Notes

There are assumptions regarding the ideal gas behavior of air and the treatment of its molecular composition that may not be universally applicable. The relationship between RMS speed and speed of sound is also contingent on the specific heat ratio, which may vary.

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TL;DR
Can we use molecular speed of gas formula also for the air? I mean if we can use molecular mass of the air.
Molecular speed of gas is generally $$v=\sqrt{\frac{3RT}{M}},$$where R is gas constant and M is molecular mass. Can we use this formula for the air as well, when the air is a mixture of different gasses?
 
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You can use the molecular speed of gas for air.
I believe the computed RMS velocity gives you the speed of sound.

The molecular weight of air is close to 28.96 g/mole.
Given single percentages and integer atomic weights.
N2 + O2 + Ar = (14*2)*0.78 + (16*2)*0.21 + (40)*0.01 = 28.96 g/mole.
 
You can model the air as a diatomic ideal gas and get some good results. Air is made mostly of nitrogen and oxygen, both of which are diatomic gasses.
 
Baluncore said:
I believe the computed RMS velocity gives you the speed of sound.
For a given gas, the two are proportional.
https://physics.stackexchange.com/questions/78879/simple-explanation-of-relation-between-speed-of-sound-and-r-m-s-speed said:
In an ideal gas, the speed of sound ##v_s## is related to the r.m.s. molecular speed ##v_m## by $$\frac{v_s}{v_m}=\sqrt{\frac{\gamma}{3}}$$where ##\gamma## = ##C_p/C_v## = 7/5 for a diatomic gas.
The article above goes on to provide an explanation for the relationship.
 

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