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Amadeo Avogadro (1776-1856)Avogadro's Law (Avogadro's theory; Avogadro's hypothesis) is a principle stated in 1811 by the Italian chemist Amedeo Avogadro (1776-1856) that "equal volumes of gases at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties". This number (Avogadro's number) is 6.022 X 1023. It is the number of molecules of any gas present in a volume of 22.41 L and is the same for the lightest gas (hydrogen) as for a heavy gas such as carbon dioxide or bromine.

The law can be stated mathematically

.

where:

V is the volume of the gas.
n is the amount of substance of the gas.
k is a proportionality constant.

The most important consequence of Avogadro's law is that the ideal gas constant has the same value for all gases. This means that the constant

where:

p is the pressure of the gas
T is the temperature of the gas

has the same value for all gases, independent of the size or mass of the gas molecules.

One mole of an ideal gas occupies 22.4 liters (dm³) at STP, and occupies 24.45 litres at SATP (Standard Ambient Temperature and Pressure = 273K and 1 atm or 101.325 kPa). This volume is often referred to as the molar volume of an ideal gas. Real gases may deviate from this value.

Or to put it another way "the principle that equal volumes of all gases at the same temperature and pressure contain the same number of molecules. Thus, the molar volume of all ideal gases at 0° C and a pressure of 1 atm. is 22.4 liters"

Avogadro's number is one of the fundamental constants of chemistry. It permits calculation of the amount of pure substance (mole), the basis of stoichiometric relationships. It also makes possible determination of how much heavier a simple molecule of one gas is than that of another, as a result the relative molecular weights of gases can be ascertained by comparing the weights of equal volumes.

Avogadro's number (conventionally represented by N' in chemical calculations) is now considered to be the number of atoms present in 12 grams of the carbon-12 isotope (one mole of carbon 12) and can be applied to any type of chemical entity.

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