The gas will obey Boyles Law:
P1 V1 = P2 V2 where P1 and V1 are the original pressure and volume and P2 and V2 are the new values.
If V2 = 2V1 (given) then:
P1 V1 = P2 *2 V1
P2 = P1 V1 / 2V1
P2 = P1 / 2
In other words the pressure is halved. (answer).
<span>The best reason I can think of for why we believe that air is a mixture is that freezing air slowly yields different liquids at different temperatures. Liquid nitrogen has a different boiling point than liquid oxygen. They also freeze at different temperatures. If air were only 1 compound, then air in its entirety would have a single boiling point and a single freezing point. </span>
