Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
Treat steam as a gas of rigid, bent molecules. Find , and . Tables give of steam near as about . Is the rigid model good? Take .
From equipartition, , and . A polyatomic gas with active vibrations has . A mixture has . A solid has , so .
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Degrees of freedom. .
Step 2. Heat capacities. . .
Step 3. Gamma. , which is .
Check. The measured is a little more than . The model is good. A small share of the energy already goes into vibration.
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