Amedeo Avogadro (1776-1856)

Amedeo Avogadro was born in Turin, Italy, on 9th August 1776, into a family of distinguished lawyers. He was himself guided towards establishing a successful legal career. However, Avogadro also showed an interest in natural philosophy, and in 1800 began private studies of mathematics and physics. In 1820, when the very first chair of mathematical physics in Italy was established at the University of Turin, Avogadro was appointed. In 1811 he published his hypothesis, now called Avogadro's law. This was ignored for the next fifty years by the scientific community of the early nineteenth century. It was Stanislao Cannizzaro (1826-1910), also an Italian chemist, who was the first to appreciate the importance of Avogadro's work. Avogadro died on the 9th July 1856.

The Law of Combining Volumes and Avogadro's Law

Experimental work on gases and their chemical reactions fascinated many scientists, among them Joseph Louis Gay-Lussac, who made some remarkable observations. These are expressed as Gay-Lussac's Law of Combining Volumes (1808) which states: When gases react, the volumes consumed and produced, measured at the same temperature and pressure, are in ratios of small whole numbers. Here are some examples:

Hydrogen gas + Oxygen gas ® Water vapour
2 dm³ + 1dm³ ® 2 dm³

Hydrogen gas + Nitrogen gas ® Ammonia gas
3 dm³ + 1 dm³ ® 2 dm³

Something had now to be made of this observation. In the early 1800s, the chemical formulas of even simple substances like hydrogen, chlorine, and water were not known with certainty. It was in 1811 that Amedeo Avogadro made the first clear statement of this relationship. In doing so he coined the term molecule, by which he meant an aggregation of atoms. In modern terms, Avogadro's Law says, Equal volumes of all gases at the same temperature and pressure contain the same number of molecules.

Expressed as an equation, where N is the number of molecules, Avogadro's Law is:

N µ V or N/V = constant (temperature and pressure are fixed)

His thought was that gas volumes must have something to do with molecules, because molecules also reacted and are formed in ratios of small whole numbers. Consider, for example, the relationships of gas volumes in the reaction between hydrogen gas and chlorine gas to produce hydrogen chloride gas.

Hydrogen gas + Chlorine gas ® Hydrogen Chloride gas
1 dm³ + 1 dm³ ® 2 dm³

The simplest possible formula for hydrogen chloride is HCl. If this formula is correct, 1 molecule of hydrogen chloride would contain 1 atom each of hydrogen and chlorine. Likewise, 2 molecules of hydrogen chloride would contain 2 atoms of hydrogen and 2 atoms of chlorine. But note from the above equation that 2 atoms of hydrogen come from only one molecule of hydrogen. The hydrogen molecule must contain 2 atoms of hydrogen and its formula must be H₂. The same reasoning leads to the formula Cl₂ for chlorine. Avogadro's Law therefore lets us select molecules of different gases in equal numbers. The law is not strictly true, except at very low pressures, or for ideal gases, but is sufficient for most simple calculations involving gases.

Molar Volume of a gas

You may recall from earlier work that a mole is the amount of any substance that contains as many particles as there are carbon atoms in 12 g of carbon-12. Since the particles of gases are molecules, a mole of a gas contains as many molecules as there are carbon atoms in 12 g of carbon-12. Since equal numbers of molecules of all gases occupy the same volume (at constant temperature and pressure), the volume of one mole of any gas must be a standard quantity. This volume, at 273 K (0 °C) and 1 atm, has been determined to be 22.4138 dm³. The number of molecules in a mole, designated N_A (also L), is called the Avogadro constant. Its value can be found by experiment and is 6.023 x 10²³ mol^-1. This is an enormous number; if we were able to count particles (atoms, molecules, ions, electrons) at the rate of 10 million per second, it would take about 2 billion years to count the particles in one mole.

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