All MCAT Physical Resources
Example Question #1 : Using Molar Volume
What is the volume of two moles of diatomic nitrogen gas at STP?
One mole of gas has a volume of 22.4L at STP.
Since there are two moles of gas present, the volume is doubled.
2(22.4) = 44.8L
The fact that the gas is diatomic is irrelevant.
Example Question #2 : Using Molar Volume
Air is composed of about 78% nitrogen (molecular mass = 28 g/mole), 21% oxygen (molecular mass = 32 g/mole), and 1% argon (40 g/mole). The weighted average of these numbers leads to the approximation that if air was a pure gas, its “molar mass” would be about 29 g/mole. Helium has a molar mass of 4 g/mole. What is the volume of helium gas required to lift a balloon carrying a 100 gram sensor?
- about 20 liters
- about 50 liters
- about 90 liters
- about 120 liters
- about 166 liters
Choice 3 is correct. The question forces the respondent to recall that one mole of an ideal gas occupies 22.4 liters at standard temperature (zero Celsius) and pressure (1 atmosphere). Helium balloons rise because they are less dense than air by a factor of 29 g/mole – 4 g/mole = 25 g/mole. Since the sensor has a mass of 100 g, it will require about 4 molar volumes of He to lift it.