# Most probable speed of aluminium atoms

by Bugsy23
Tags: aluminium, atoms, probable, speed
 P: 25 1. The problem statement, all variables and given/known data Aluminium atoms at 1173K enter a vacuum chamber. Calculate the most probable speed for the atoms. 2. Relevant equations vmp=$$\sqrt{}$$(2kT/m) 3. The attempt at a solution The molar mass of aluminium is 0.0270kg mol^-1, so vmp=$$\sqrt{}$$(2*1.381*10^-23JK^-1)*1173K/0.0270kg mol^-1=1.1*10^-9ms^-1 Which is a really tiny quantity and it seems unrealistic that particles at such a high temperature would move that slowly. I've used SI units though so I can't see where I would have gone wrong. Any help would be appreciated. Thanks
PF Patron
HW Helper
P: 1,685
 Quote by Bugsy23 The molar mass of aluminium is 0.0270kg mol^-1, so vmp=$$\sqrt{}$$(2*1.381*10^-23JK^-1)*1173K/0.0270kg mol^-1=1.1*10^-9ms^-1
You're using the molar mass combined with Boltzmann constant. But that doesn't make sense. You need the mass of an individual molecule (in this case an individual aluminum atom) if you want to use Boltzmann constant. (You can do this by dividing the molar mass by Avogadro constant NA to find the mass per individual molecule [in this case atom].)

Alternatively, you can stick with the molar mass and substitute Boltzmann constant k with the gas constant R, since k = R/NA.
 P: 25 Thanks, I've got it now

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