STANDARD Physics Heat Capacities MCQ Question

Water is treated like a solid. For each atom, the average energy is $3k_B T$. A water molecule has three atoms, so the total energy of one mole of water is $U = 3 \times 3k_B T \times N_A = 9RT$. Thus, the heat capacity per mole of water is $C = \frac{\Delta Q}{\Delta T} = \frac{\Delta U}{\Delta T} = 9R$.