Calculation of average molecular speed

  • #1
subhradeep mahata
120
13
Homework Statement:
Calculate the average speed of 2 mol oxygen molecules at pi*10bar pressure occupying 8 litres volume.
Relevant Equations:
Given in the "Attempt at a Solution" section
We can calculate the average speed using this formula: uavg=root(8PV/ΠnM)
I plugged in the following values:
P= pi*10 atm (bar is approximately equal to atm)
V=8lts
n=2mol
M=32*10^-3kg/mol
But it is giving wrong answer. Can you tell me what went wrong?
 

Answers and Replies

  • #2
HallsofIvy
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Is this a joke or do you just want someone to do your homework for you? You ask "what went wrong" but don't show what you did nor what answer you got!
 
  • #3
mjc123
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Units?
 
  • #4
subhradeep mahata
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@HallsofIvy Actually I am pretty sure that I plugged in values having wrong units, that's why I just showed the values and what formula I am using. I just wanted to know what units are to be used in the formula.
 
  • #5
subhradeep mahata
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@mjc123 Yeah probably. Can you tell me what units should I use?
 
  • #6
mjc123
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Consistent ones. I would convert everything to SI units.

The one thing you must never do is blindly plug numbers into a formula without consideration of units, e.g. mixing atm, L, kg etc. (some SI and some not).
 
  • #7
Charles Link
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Suggest you use SI units. The pressure ## P ## needs to be in ## Newton/m^2 ##. ## \\ ## The formula you have comes from ## \bar{v}=\sqrt{\frac{8 k_bT}{\pi m}} ##, where ## m ## is the mass of the molecule or atom. ##\\ ## Meanwhile, with the ideal gas law ## PV=n R T=N k_b T ##, where ## N=## number of molecules, so that ## N=n N_A ## where ## n =## number of moles, and ## N_A=## Avagadro's number, we can write ## \bar{v}=\sqrt{\frac{8 PV}{\pi n M}} ##, where ## M=m N_A ##, is the mass of a mole of atoms or molecules.
 
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