At constant temperature, a sample of helium at 760. torr in a closed container was compressed from 5.00 l to 3.00 l, with no change in moles or temperature. what was the new pressure exerted by the helium on its container?

To solve this problem, we will use Boyle's law which states that: 
At constant temperature, P1V1 = P2V2 where:
P1 is the initial pressure = 760 torr
V1 is the initial volume = 5 liters
P2 is the final pressure that we want to calculate
V2 is the final volume = 3 liters

Substitute in the above equation to get P2 as follows:
(760)(5) = P2 (3)
P2 = (760*5) / (3) = 1266.667 torr

The density would be 1.622