Which of the following gases is considered non-condensable under standard conditions?

• A. CO_2
• B. NH_3
• C. N_2
• D. H_2O vapor

Answer: (C)

Verified step by step guidance

1. Understand the term 'non-condensable gas': A non-condensable gas is one that does not easily change from the gas phase to the liquid phase under standard temperature and pressure (STP) conditions, meaning it has a very low tendency to condense.
2. Review the properties of each gas under standard conditions (usually 1 atm pressure and 25°C): Consider their boiling points and whether they exist as gases or liquids at these conditions.
3. Check the boiling points of the gases: CO_2 sublimates at -78.5°C, NH_3 boils at -33.3°C, H_2O vapor condenses at 100°C, and N_2 boils at -195.8°C. Since standard conditions are much warmer than these boiling points, gases with higher boiling points tend to condense more easily.
4. Identify which gases are likely to condense: CO_2, NH_3, and H_2O vapor have relatively higher boiling points compared to N_2, so they can condense under standard conditions or slightly below, while N_2 remains gaseous and is considered non-condensable.
5. Conclude that N_2 is the non-condensable gas under standard conditions because it remains in the gas phase and does not easily liquefy at these conditions.