What are rarefactions and compressions of sound waves?

At 300 mm hg, a gas has a volume of 380 l, what is the volume at standard pressure

Consider this initial-rate data at a certain temperature for the reaction described by

Which best describes the present-day universe? opaque, expanding very slowly, stars produce heavy elements transparent, expanding at an accelerated rate, stars produce heavy elements opaque, expanding at an accelerated rate, stars produce only hydrogen and helium transparent, expanding very slowly, stars produce only hydrogen and helium