# 9: Gases

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Of the three basic phases of matter—solids, liquids, and gases—only one of them has predictable physical properties: gases. In fact, the study of the properties of gases was the beginning of the development of modern chemistry from its alchemical roots. The interesting thing about some of these properties is that they are independent of the identity of the gas. That is, it doesn’t matter if the gas is helium gas, oxygen gas, or sulfur vapors; some of their behavior is predictable and very similar. In this chapter, we will review some of the common behaviors of gases. Gases have no definite shape or volume; they tend to fill whatever container they are in. They can compress and expand, sometimes to a great extent. Gases have extremely low densities, a one-thousandth or less of the density of a liquid or solid. Combinations of gases tend to mix together spontaneously—that is, they form solutions. Air, for example, is a solution of mostly nitrogen and oxygen. Any understanding of the properties of gases must be able to explain these characteristics.

• 9.1: Kinetic Molecular Theory- A Model for Gases
The physical behavior of gases is explained by the kinetic theory of gases. An ideal gas adheres exactly to the kinetic theory of gases.
• 9.2: Pressure - The Result of Constant Molecular Collisions
Pressure is a force exerted over an area. Pressure has several common units that can be converted.
• 9.3: Boyle’s Law - Pressure and Volume
Boyle’s Law relates the pressure and volume of a gas at constant temperature and amount.
• 9.4: Charles’s Law- Volume and Temperature
Charles’s Law relates the volume and temperature of a gas at constant pressure and amount. In gas laws, temperatures must always be expressed in kelvins.
• 9.5: The Combined Gas Law- Pressure, Volume, and Temperature
There are gas laws that relate any two physical properties of a gas. The Combined Gas Law relates pressure, volume, and temperature of a gas.
• 9.6: Avogadro’s Law- Volume and Moles
The original statement of Avogadro’s law states that equal volumes of different gases at the same temperature and pressure contain the same number of particles of gas. Because the number of particles is related to the number of moles, Avogadro’s law essentially states that equal volumes of different gases at the same temperature and pressure contain the same amount (moles, particles) of gas.
• 9.7: The Ideal Gas Law- Pressure, Volume, Temperature, and Moles
The Ideal Gas Law relates the four independent physical properties of a gas at any time. The Ideal Gas Law can be used in stoichiometry problems with chemical reactions involving gases. Standard temperature and pressure (STP) are a useful set of benchmark conditions to compare other properties of gases. At STP, gases have a volume of 22.4 L per mole. The Ideal Gas Law can be used to determine densities of gases.
• 9.8: Gay-Lussac's Law- Temperature and Pressure
Gay-Lussac's Law states that the pressure of a given mass of gas varies directly with the absolute temperature of the gas, when the volume is kept constant. Gay-Lussac's Law is very similar to Charles's Law, with the only difference being the type of container. Whereas the container in a Charles's Law experiment is flexible, it is rigid in a Gay-Lussac's Law experiment.
• 9.9: Mixtures of Gases - Dalton's Law of Partial Pressures
The pressure of a gas in a gas mixture is termed the partial pressure. Dalton’s Law of Partial Pressures states that the total pressure in a gas mixture is the sum of the individual partial pressures. Collecting gases over water requires that we take the vapor pressure of water into account. Mole fraction is another way to express the amounts of components in a mixture.
• 9.10: Gases in Chemical Reactions
Many reactions that are carried out in the laboratory involve the formation or reaction of a gas, so chemists must be able to quantitatively treat gaseous products and reactants as readily as they quantitatively treat solids.  With the ideal gas law, we can use the relationship between the amounts of gases (in moles) and their volumes (in liters) to calculate the stoichiometry of reactions involving gases, if the pressure and temperature are known.

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