For gaseous coolants used in nuclear or process heat removal (H2, He, CO2), arrange their specific heats (on a mass basis, kJ/kg-K) in increasing order for quick thermal comparison.

Introduction / Context:
Engineers often compare candidate gaseous coolants by their ability to carry heat per unit mass and per unit volume. A fast way to benchmark thermal performance is to order fluids by specific heat at constant pressure, Cp, usually expressed on a mass basis (kJ/kg-K) for pumping and equipment-sizing calculations. This question asks for the increasing order for three common gases, carbon dioxide (CO2), helium (He), and hydrogen (H2), as used in high-temperature nuclear and process systems.

Given Data / Assumptions:

• We compare Cp on a mass basis (kJ/kg-K) at near-ambient conditions where standard reference data are tabulated.
• Ideal-gas behavior is assumed for ranking; modest temperature variation does not change the relative order.
• Target fluids: CO2, He, H2.

Concept / Approach:
On a molar basis, monatomic gases like He have Cp ≈ (5/2)R, diatomic like H2 have Cp ≈ (7/2)R near room temperature, and polyatomic like CO2 have even higher molar Cp because of additional degrees of freedom. However, when expressed per kilogram, molecular weight matters strongly. Hydrogen has a tiny molecular weight, so dividing its molar Cp by 2 g/mol yields a very large kJ/kg-K. Helium, with molecular weight 4 g/mol, still yields a high mass-based Cp. Carbon dioxide, with molecular weight 44 g/mol, ends up with a comparatively low mass-based Cp.

Step-by-Step Solution:
1) Recall approximate mass-based Cp values: CO2 ≈ 0.84–0.90 kJ/kg-K; He ≈ 5.1–5.2 kJ/kg-K; H2 ≈ 14–14.5 kJ/kg-K.2) Rank in increasing order by value: smallest first → CO2, then He, then H2.3) Match to choices: CO2, He, H2.

Verification / Alternative check:
Convert typical molar Cp to mass basis. Example: Cp,He ≈ 20.8 J/mol-K, divide by 4 g/mol → ~5.2 kJ/kg-K; Cp,H2 ≈ 28.8 J/mol-K, divide by 2 g/mol → ~14.4 kJ/kg-K; Cp,CO2 ≈ 37 J/mol-K, divide by 44 g/mol → ~0.84 kJ/kg-K. The mass-basis order is consistent across normal temperature ranges for gas cooling applications.

Why Other Options Are Wrong:
CO2, H2, He: incorrectly places He above H2 on a mass basis.H2, He, CO2 and H2, CO2, He: start with H2, which actually has the highest, not the lowest, mass-based Cp.

Common Pitfalls:
Mixing molar and mass bases; ignoring the impact of molecular weight on mass-specific properties; assuming high molar Cp necessarily means high mass Cp without conversion.

Final Answer:
CO2, He, H2