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Chemistry and Gases Study Guide

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Updated on Sep 24, 2011

Introduction

Many gases are colorless substances we take for granted. They continually surround us and supply us with needed oxygen and supply plants with needed carbon dioxide. Gases' unique property of compressibility allows for quick and observable changes. Changes in pressure, volume, temperature, and other physical properties can be calculated with various ideal gas laws.

Properties of Gases

Gases are fluid, compressible substances. All gases behave according to the following characteristics in the kinetic molecular theory (KMT):

  • Gas particles expand to assume the volume and shape of their container.
  • The volume of gas particles is assumed to be negligible.
  • Gas particles are in constant motion.
  • Gas particles mix evenly and completely when confined in the same container.
  • Gas particles collide with each other; they do not attract or repel each other, and they do not exert a force on each other.
  • The average kinetic energy of gas molecules is proportional to the temperature (in Kelvin) of the gas. The higher the gas temperature, the higher the kinetic energy.

Because gases are compressible, they exert pressure on their surroundings. Pressure is the force that is exerted over a unit area. For example, the atmosphere exerts a pressure known as atmospheric pressure. The Earth's atmosphere is a function of the location and the weather conditions, and it decreases with a higher altitude. The unit of pressure commonly used in chemistry is the atmosphere (atm). The standard atmosphere is 1 atm or a measurement of 760 millimeters of mercury (mm Hg or torr) on a manometer.

1 atm = 760 mm Hg = 760 torr = 101,325 Pa (pascal or N/m2)

Note

The most common atmospheric pressure unit used by meteorologists is inches of mercury in Hg (760 mm Hg = 29.92 in Hg).

Example:

Covert 830 mm Hg to atmospheres.

Solution:

Boyles's, Charles's, Gay- Lussac's, and Avogadro's Laws

Properties of gases that change are pressure (P), temperature (T), volume (V), and the number of moles (n). Several laws relate these properties:

  • Boyle's Law (at constant temperature): The volume of a gas (maintained at constant temperature) decreases as its pressure increases (P α1/V):
  • P1V1 = P2V2

  • Charles's Law (at constant pressure): The volume of a gas (maintained at constant pressure) increases directly with an increase in its Kelvin temperature (V α T):
  • Gay-Lussac's Law (at constant volume): The pressure of a gas (maintained at constant volume) increases with an increase in its Kelvin temperature (P α T):
  • Avogadro's Law (at constant T and P): The volume of gas increases with the number of moles of gas present at constant temperature and pressure.(Vα n)

The standard temperature and pressure (STP) condition is achieved at 273.15 K and 1 atm (760 torr) when one mole (or 6.02 ×1023 particles or molecules) of any gas occupies a volume of 22.4 liters (molar volume at STP).

Note

It is important in all the gas law problems that you make sure the answer makes sense. In the previous example, the temperature is increasing, and Charles's Law states that volume increases with an increase in temperature (at constant pressure).

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