JEE Main 2026 — Thermodynamics (C) Question with Solution

The molar heat capacity of Helium (a monatomic gas) at constant pressure is $C_p=\frac{5}{2}R\approx 20.8{\text{ J K}}^{-1}{\text{ mol}}^{-1}$.

The molar heat capacity of Copper (a solid) follows the Dulong-Petit law, which gives $C_p\approx 3R\approx 24.9{\text{ J K}}^{-1}{\text{ mol}}^{-1}$.

The molar heat capacity of Bromine (a diatomic liquid) is significantly higher due to translational, rotational, and vibrational degrees of freedom along with intermolecular interactions in the liquid state, having $C_p\approx 75.7{\text{ J K}}^{-1}{\text{ mol}}^{-1}$.

Therefore, the correct order of molar heat capacities is Bromine(l) > Copper(s) > Helium(g).

Answer: Bromine(l) > Copper(s) > Helium(g)