MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020

The Gas Phase

Chapter Profile


The content in this chapter should be relevant to about 8% of all questions about general chemistry on the MCAT.

This chapter covers material from the following AAMC content categories:

3B: Structure and integrative functions of the main organ systems

4B: Importance of fluids for the circulation of blood, gas movement, and gas exchange


Let's start this chapter with a thought experiment. Imagine a helium balloon tied to the gearshift lever between the seats of your car and allowed to float freely. What do you think will happen to the balloon as you accelerate forward? You might think, based on how you feel when you are in an accelerating car, that the balloon will be pushed backwards due to its inertia. However, the balloon’s movement isn’t what we might predict: the balloon shifts forward as the car accelerates!

The molar mass of helium is Image while that of air, which is mostly nitrogen and oxygen, is about Image This means that air is about seven times denser than helium. Because the air in which the balloon is floating is more dense than the balloon itself, the air has greater inertia. Therefore, as the car accelerates forward, everything that has significant mass, including the air in the car, resists the forward motion (has inertia) and shifts toward the back of the car (even though, of course, everything in the car is accelerating forward, just not as quickly as the car itself). As the air shifts toward the back, a pressure gradient builds up such that there is greater air pressure in the back of the car than in the front, and this pressure difference results in a pushing force against the balloon that is directed from the back toward the front. Responding to this force, the balloon shifts forward in the direction of the car’s acceleration. Who would have thought that general chemistry and physics could be so much fun?

In this chapter, we will discuss some MCAT favorites—the gas phase and the ideal gas laws. We will begin our discussion with ideal gases and the laws that govern their behavior. We will then examine the kinetic molecular theory that describes ideal gases and conclude with an evaluation of the ways in which the behavior of real gases deviates from that predicted by the ideal gas law.