# Calculation of average molecular speed

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?

Homework Helper
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!

Homework Helper
Units?

@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.

@mjc123 Yeah probably. Can you tell me what units should I use?

Homework Helper
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).

Homework Helper
Gold Member
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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