MCAT General Chemistry Review

Chapter 8: The Gas Phase


The next time you’re leaving a birthday party, snag a helium balloon on your way out. Tie the helium balloon to the gearshift lever between the front seats, making sure that the balloon is floating freely. Once you’re on an open road, accelerate the car abruptly, and as you do, watch the balloon—safely! What do you predict will happen to the balloon as the car accelerates? What do you observe? You might think, based on how you feel when you are sitting in a car accelerating forward, that the balloon will be pushed toward the back of the car due to its inertia. However, the balloon’s movement isn’t what we might predict: the balloon shifts forward as the car accelerates because the balloon is filled with helium. The molar mass of helium is  while that of air, which is mostly nitrogen and oxygen, is about  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 will have greater inertia. In fact, we can approximate the balloon’s inertia as practically nonexistent. 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 of 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.