Well, I only know that the answer is Cv= 3R or 3Nk. I don't get it at all... The exercise I have to derive it using the kinetic theory of gases and the equipartition of energy. My professor said it was not that hard and all the equations were on the web, but I have no idea how this works and which equations I need. I assume the kinetic theory of gases gives me PV=NkT but I don't know how equipartition of energy works.
Problem is, I can't use any quantum mechanics, I have to do this the classical way. It's a crystalline solid, by the way. I've found that the energy for an atom is k*T/2, and that it depends on the degrees of freedom for that atom. Now I've read that the DoF should be 6, I don't really know why other than that it has something to do with it being a crystalline solid (no translation or rotation). Then the molar internal energy U should be U=6*N*k*T/2, because of the six degrees of freedom and Avogadro's constant K. Then Cv= dU/dT with constant V, thus being 3Nk, but it surely couldn't be that simple? Where's the kinetic theory of gases in this?