Orodruin said:
Your Cv in the relevant equations has the wrong physical dimension to be specific heat. What you have written has dimension energy per temperature per substance amount, not the correct energy per temperature per mass.
Thank you Orodruin and Charles Link,
Orodruin, the book I took the problem from (Halliday & Resnick, 1966) says, in this context,
By definition of ##C_v## we have ##\Delta Q = \mu C_{v} \Delta T##
They are using ##\mu## for the number of moles.
This gives C
v dimensions ML
2T
-3 or as you say 'energy per temp per quantity of substance amount'.
So, I could not agree with you that..
Your Cv in the relevant equations has the wrong physical dimension to be specific heat
Different yes, but not wrong.
But the statement of the problem gave ##C_{v} = 0.075 kcal/kg K##.
And I complained I couldn't isolate the mass.
And your suggested 'energy per temperature per mass' had 'mass' in it.
Hmmm, your suggestion was not just different; it might be
useful.
So, is this what you were suggesting?...
From kinetic theory and for a mole of a monoatomic gas like Ar,$$U=\frac 1 2 M\bar{ v^{2}} = \frac 3 2 RT$$ and now, with your suggestion$$U =MC_{v}T$$I got$$M=3R/2C_{v}$$
Plugging in the given ##C_v## and ##R## (converted to the same units as were given for ##C_v## gives about the molecular weight (in kg) for Ar.
The analogous calculation starting the average translational kinetic energy per molecule$$U=\frac 1 2 m\bar{ v^{2}} = \frac 3 2 kT$$gives$$m=3k/2C_v$$for the mass of the Ar atom. And plugging in ##k## in the same units as were given for ##C_v## gives a good value for the mass of the Ar atom.
(Could also divide the molecular weight by Avogadro's number to get the mass of the atom, but the topic is kinetic theory.)