A gas has a volume of 400 mL at STP. If the temperature is held constant, what volume would the gas occupy at room temperature of 1.5 atm?
If you assume idea gas behaviour, then 266.67 ml.
If you assume that it behaves as an ideal gas, then you can use the relationship Pv = nRT. Therefore, v = (nRT)/P. Therefore, given that n, R and T are all constant, the volume will simply vary inversely with pressure.
Pressure at STP is 1 atm, so the pressure will decrease by 1/1.5 which is (1/1.5) x 400 = 266.67 ml.