this is probably too late a response for you, but I'll post it for future readers:
The Sackur-Tetrode equation is:
S = Nk (ln \left(\frac{4V^{2/3} \pi mU}{3N^{5/3}h^2}\right) +5/2)
Just take the derivative with respect to U:
\frac{\partial S}{\partial U} = \frac{1}{T} =...