Avogadro's Law of Gas

The behavior of gases described by Boyle’s, Charles’s, and Dalton’s laws is nearly the same for all gases. Avogadro’s law states that under identical conditions of temperature and pressure, equal volumes of different gases contain equal numbers of particles (atoms or molecules).

The temperature 0ºC (32ºF) and the pressure equal to the pressure Earth’s atmosphere exerts at sea level are called standard temperature and pressure (STP). According to Avogadro’s law, 1 cubic meter of oxygen at STP contains the same number of particles as 1 cubic meter of nitrogen at STP. Restated, Avogadro's law says that one mole of any gas at STP occupies a volume of 22.4 liters. A mole is basic particles (atoms or molecules) of a substance. The extremely large number is called Avogadro's number.

Dalton's Law of Gas

Dalton’s law states that in a mixture of different gases, such as air, the sum of the partial pressures of all the gases equals the total pressure. The partial pressure of a gas is the pressure that gas would exert if it was the only gas present. This law, expressed mathematically, is:

where each subscripted P value is the partial pressure of a different gas.

Air, for example, consists mostly of nitrogen and oxygen, with small quantities of argon, water vapor, and carbon dioxide. The partial pressure of nitrogen accounts for 78 percent of the total pressure exerted by Earth’s atmosphere; oxygen accounts for 21 percent; and argon accounts for 0.9 percent. Together, these three gases account for 99.9 percent of air pressure. Water vapor, carbon dioxide, and all the other trace gases present in the atmosphere combined contribute only a tenth of a percent.

Charles’s Law of Gas

French chemist Jacques Charles discovered that the volume of a gas at constant pressure changes with temperature. As the temperature of the gas increases, so does its volume, and as its temperature decreases, so does its volume. French chemist Joseph Guy-Lussac refined Charles’s theory, and the law is also known as Guy-Lussac’s law.

Raising the temperature of a gas causes the gas to fill a greater volume as long as pressure remains constant. Gases expand at a constant rate as temperature increases, and the rate of expansion is similar for all gases. Charles's law (also called Gay-Lussac’s law) describes the relationship between volume and temperature of an enclosed gas. The law says that at constant pressure, the volume of a fixed number of particles of gas is directly proportional to the absolute (Kelvin) temperature, mathematically expressed as:

V = k T

where T is temperature on the Kelvin scale (see Temperature: Temperature Scales). If the temperature of a given amount of gas is doubled, for example, its volume will also double (as long as pressure remains unchanged).