Amedeo Avogadro Contribution

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In 1811 Avogadro put forward a hypothesis that was neglected by his contemporaries for years. Eventually proven correct, this hypothesis became known as Avogadro’s law, a fundamental law of gases.

Avogadro’s Hypothesis

The contributions of the Italian chemist Amedeo Avogadro (1776–1856) relate to the work of two of his contemporaries, Joseph Louis Gay-Lussac and John Dalton.Gay-Lussac’s law of combining volumes (1808) stated that when two gases react, the volumes of the reactants and products—if gases—are in whole number ratios. Lorenzo Romano Amedeo Carlo Avogadro (August 9, 1776 to July 9, 1856) Avogadro is famous for Avogadro’s Law, which states that two gases of equal volume at the same temperature and pressure contain an equal number of molecules. In honor of him, the number of molecules in a mole of a substance is called Avogadro’s number. Avogadro, Amedeo, Conte di Quaregna (1776–1856) Italian physicist and chemist. His hypothesis, Avogadro's law (1811), states that equal volumes of gases at the same pressure and temperature contain an equal number of molecules.

In 1811 Avogadro hypothesized that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. From this hypothesis it followed that relative molecular weights of any two gases are the same as the ratio of the densities of the two gases under the same conditions of temperature and pressure. Avogadro also astutely reasoned that simple gases were not formed of solitary atoms but were instead compound molecules of two or more atoms. (Avogadro did not actually use the word atom; at the time the words atom and molecule were used almost interchangeably. He talked about three kinds of “molecules,” including an “elementary molecule”—what we would call an atom.) Thus Avogadro was able to overcome the difficulty that Dalton and others had encountered when Gay-Lussac reported that above 100°C the volume of water vapor was twice the volume of the oxygen used to form it. According to Avogadro, the molecule of oxygen had split into two atoms in the course of forming water vapor.

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Edgar Fahs Smith Collection, Kislak Center for Special Collections, Rare Books and Manuscripts, University of Pennsylvania

Amedeo Avogadro Discovery

Curiously, Avogadro’s hypothesis was neglected for half a century after it was first published. Many reasons for this neglect have been cited, including some theoretical problems, such as Jöns Jakob Berzelius’s “dualism,” which asserted that compounds are held together by the attraction of positive and negative electrical charges, making it inconceivable that a molecule composed of two electrically similar atoms—as in oxygen—could exist. In addition, Avogadro was not part of an active community of chemists: the Italy of his day was far from the centers of chemistry in France, Germany, England, and Sweden, where Berzelius was based.

Personal Life

Avogadro was a native of Turin, where his father, Count Filippo Avogadro, was a lawyer and government leader in the Piedmont (Italy was then still divided into independent countries). Avogadro succeeded to his father’s title, earned degrees in law, and began to practice as an ecclesiastical lawyer. After obtaining his formal degrees, he took private lessons in mathematics and sciences, including chemistry. For much of his career as a chemist he held the chair of physical chemistry at the University of Turin.

The information contained in this biography was last updated on November 30, 2017.

Avogadro's hypotheses

Amedeo Avogadro's principal contribution to chemistry was a paper in which he advanced two hypotheses: (1) that equal volumes of gas contain equal numbers of molecules and (2) that elementary gases such as hydrogen, nitrogen, and oxygen were composed of two atoms.[1] For simplicity, let us call the first the 'volumes' hypothesis and the second the 'diatomics' hypothesis. Avogadro was correct in both of these hypotheses, which he used to reconcile and correct Dalton's atomic hypothesis with Gay-Lussac's results on combining volumes.

1) If the volumes hypothesis is correct, then the relative masses of gas molecules can be computed from gas densities. (After all, the density is the mass of a unit volume, and that volume contains the same number of molecules, whatever the gas.) Construct a table of relative molecular masses based on the gas densities reported by Avogadro and using hydrogen as the unit molecular mass.

molecule	gas density (air = 1)
oxygen	1.10359
hydrogen	0.07321
nitrogen	0.96913
oxymuriatic acid	2.470
water vapor	0.625
nitrous gas	1.03636
nitrous oxide	1.52092
muriatic acid gas	1.278

(Hint: The molecular mass of 'nitrous gas' relative to hydrogen is the ratio of the mass of a 'nitrous gas' molecule to a hydrogen molecule.)

2) The mass scale computed in the previous exercise has the hydrogen molecule as its unit mass. It might make more sense to take the hydrogen atom as the unit. What is the mass of the hydrogen molecule if the hydrogen atom is one mass unit? On this sacle, what is the mass of the oxygen molecule, nitrous gas, etc.?

3) Hydrogen, oxygen, nitrogen, and 'oxymuriatic acid' are all diatomic elements. (Nowadays we call 'oxymuriatic acid' chlorine.) On this scale where a hydrogen atom is one mass unit, what are the atomic masses of these elements?

4) Assume that molecules are made of small whole numbers of atoms. The masses of all of the constituent atoms ought to add up to the mass of a molecule.
a) Water vapor is a compound of hydrogen and oxygen. How many atoms of each element in one molecule of water vapor? Show that the molecular mass of water vapor is consistent with the atomic masses of the atoms and your proposed formula. (For example, Dalton thought that the formula for water was HO. If that were correct, then the molecular mass of water vapor ought to equal the atomic mass of hydrogen plus that of oxygen.)
b) Nitrous gas is a compound of nitrogen and oxygen. How many atoms of each element in one molecule of nitrous gas? Show that the molecular mass is consistent with the formula.
c) Nitrous oxide is another compound of nitrogen and oxygen. How many atoms of each element in one molecule of nitrous oxide? Show that the molecular mass is consistent with the formula.
d) Muriatic acid gas is a compound of hydrogen and oxymuriatic acid. How many atoms of each element in one molecule of muriatic acid gas? Show that the molecular mass is consistent with the formula.

5) Write balanced chemical equations for the formation of water vapor, nitrous gas, nitrous oxide, and muriatic acid from their diatomic elements.

Reference

Amedeo Avogadro, 'Essay on a Manner of Determining the Relative Masses of the Elementary Molecules of Bodies, and the Proportions in Which They Enter into These Compounds,' Journal de Physique

Amedeo Avogadro Law Equation

, 58-76 (1811) [1]Actually, Avogadro proposed that these gases were composed of two parts or a multiple of two. Copyright 2003 by Carmen Giunta. Permission is granted to reproduce for non-commercial educational purposes.