Kaplan MCAT General Chemistry Review

Note that including the correction term (a) has the effect of increasing the observed pressure (P) to that predicted by the ideal gas law.

MCAT Expertise

Be familiar with the concepts embodied by this equation but do not spend too much time working with it or memorizing it, as it is not likely to be tested directly on the MCAT.

Conclusion

In this chapter, we reviewed the basic characteristics and behaviors of gases. The kinetic molecular theory of gases lays out the explanation for the behavior of ideal gases as described by the ideal gas law. The ideal gas law shows the mathematical relationship among four variables associated with gases: pressure, volume, temperature, and number of moles. We examined special cases of the ideal gas law in which temperature (Boyle’s law) or pressure (Charles’ law) is held constant. Boyle’s law shows that when temperature is held constant, there is an inverse relationship between pressure and volume. Charles’ law shows that when pressure is held constant, there is a direct relationship between temperature and volume. We also examined Dalton’s law, which relates the partial pressure of a gas to its mole fraction and the sum of the partial pressures of all the gases in a system to the total pressure of the system. Finally, we examined the ways in which real gases deviate from the predicted behaviors of ideal gases. The van der Waals equation of state is a useful equation for correcting deviations based on molecular interactions and volumes.

From helium-filled balloons to the bubbles of carbon dioxide in a glass of soda, from the pressurized gases used for scuba diving to the air we breathe on land, gases are all around us. And yet, for all the different gases that bubble, flow, and settle in and through our daily living experiences, they behave in remarkably similar ways. Expect that the MCAT will treat gases with the level of attention that is appropriate to their importance in our physical lives.

CONCEPTS TO REMEMBER

Gases are the least dense phase of matter. They are classified, along with liquids, as fluids because they flow in response to shearing forces and conform to the shape of their containers. Unlike liquids, however, gases are compressible.

The state of a gas system can be characterized by four properties: pressure, volume, temperature, and number of moles. Standard temperature and pressure (STP) is a set of conditions common in the study of gases; standard temperature is 273 K (0°C), and standard pressure is 1 atm.

Ideal gases are described by the kinetic molecular theory of gases, which characterizes gases as composed of particles with negligible volume, with no intermolecular forces, in continuous and random motion, undergoing elastic collisions with each other and the walls of their container, and having an average kinetic energy that is proportional to the temperature.

Graham’s law of diffusion and effusion states that for two or more gases at the same temperature, a gas with lower molar mass will diffuse or effuse more rapidly than a gas with higher molar mass.

Regardless of chemical identity, equal amounts of gases occupy the same volume if they are at the same temperature and pressure. For example, one mole of any gas occupies 22.4 liters at STP.

The ideal gas law, PV = nRT, describes the mathematical relationship among the four variables of the gas state for an ideal gas.

Boyle’s law is a special case of the ideal gas law for which temperature is held constant; it shows an inverse relationship between pressure and volume.

Charles’ law is a special case of the ideal gas law for which pressure is held constant; it shows a direct relationship between temperature and volume.

Dalton’s law of partial pressure states that the individual gas components of a mixture of gases will exert individual pressures, called partial pressures, in proportion to their mole fractions. The total pressure of a mixture of gases is equal to the sum of the individual partial pressures of the individual gas components.

The behavior of real gases deviates from that predicted by the ideal gas law, especially under conditions of very high pressure or very low temperature. The van der Waals equation of state is used to correct for deviations due to intermolecular attractions and molecular volumes.

EQUATIONS TO REMEMBER

Practice Questions

1. Based on your knowledge of gases, what conditions would be least likely to result in ideal gas behavior?

A. High pressure and low temperature

B. Low temperature and large volume

C. High pressure and large volume

D. Low pressure and high temperature

2. Calculate the density of neon gas at STP in g L^-1. The molar mass of neon can be approximated to 20.18 g mol^-1.

A. 452.3 g L^-1

B. 226.0 g L^-1

C. 1.802 g L^-1

D. 0.9009 g L^-1