Which of the following molecules will behave most like an ideal gas under standard temperature and pressure conditions?

• A. He
• B. H_2O
• C. CO_2
• D. NH_3

Answer: (A)

Verified step by step guidance

1. Recall that ideal gases are hypothetical gases that perfectly follow the Ideal Gas Law, \(PV = nRT\), with no intermolecular forces and negligible molecular volume.
2. Consider the nature of each molecule: Helium (He) is a noble gas with monoatomic, nonpolar particles and very weak intermolecular forces (essentially none).
3. Water (H\(_2\)O), carbon dioxide (CO\(_2\)), and ammonia (NH\(_3\)) are all polyatomic molecules with stronger intermolecular forces such as hydrogen bonding (H\(_2\)O and NH\(_3\)) or dipole-dipole interactions (NH\(_3\)), or have larger molecular sizes (CO\(_2\)).
4. Stronger intermolecular forces and larger molecular sizes cause deviations from ideal gas behavior because they affect how molecules attract or repel each other and occupy space.
5. Therefore, the molecule that behaves most like an ideal gas under standard temperature and pressure is the one with the weakest intermolecular forces and smallest molecular size, which is helium (He).