What is the internal energy of an ideal gas at room tempature?

Why is methane gas at room tempature and octane a liquid at room temperature?

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• Answer:

  Annie It is because molecular size matters. Very small molecules like methane (5 atoms) similar in size to the molecules in air (N2, O2, CO2) are easily agitated and carried away into the air. In fact it takes confinement and pressure to keep methane in one place. Under pressure methane can become a liquid, it can even freeze into a solid. Larger molecules like octane (26 atoms) are very big and bulky. Big molecules can not float on little air molecules. In fact the air molecules pressing down on octane keep it from becoming a gas. If you remove the air above octane (by say pulling a vacuum and sucking out most of the air) all of a sudden octane becomes a gas too.