The law of conservation of mass
This law states that in a reaction, the total mass of the reactants and the total mass products will be the same. Therefore, no mass is lost or created during a reaction, hence it is conserved. So for example, if calcium carbonate is thermally decomposed to give 28 grams of calcium oxide and 22 grams of carbon dioxide, then the initial amount of calcium carbonate would be exactly 50 grams (assuming that no reactants escaped and the reaction was perfect). Here are some reasons as to why this phenomenon may not always be observed:
- reversible reactions may not go to completion
- some product may be lost when it is removed from the reaction mixture
- some of the reactants may react in an unexpected way
Antoine Lavoisier is credited for writing the law of conservation of mass. He wrote:
"Nothing is created, either in the operations of art or in those of nature, and it may be considered as a general principle that in every operation there exists an equal quantity of matter before and after the operation; that the quality and quantity of the constituents is the same, and that what happens is only changes, modifications. It is on this principle that is founded all the art of performing chemical experiments; in all such must be assumed a true equality or equation between constituents of the substances examined, and those resulting from their analysis."
However, many scientists had already outlined the ideas of The Law of Conservation of Mass before, but he did independently discovered it and wrote, researched and taught the law the most extensively.
Pre-history leading up to Lavoisier
Anaxagoras in 450 B.C. said:
"Wrongly do the Greeks suppose that ought begins or ceases to be; for nothing comes into being or is destroyed; but all is an aggregation or secretion of pre-existing things; so that all becoming might more correctly be called becoming mixed, and all corruption, becoming separate."
Around 1623, Francis Bacon wrote:
"Men should frequently call upon nature to render her account; that is, when they perceive that a body which was before manifest to the sense has escaped and disappeared, they should not admit or liquidate the account before it has been shown to them where the body has gone to, and into what it has been received."
Joseph Black made extensive studies of the carbonates of the alkali and alkaline earth metals, and while doing so, he noted that the additional mass of the product was probably because the reactants were reacting with air too. Therefore, he hinted that the mass of the reactant should be the same as the mass of the products, in a perfect experiment. Henry Cavendish was also a scientist who seems to have had an understanding of the Law of Conservation of Mass but did not explicitly state it.
When John Dalton came up with his atomic model, he theorised that all atoms of a given element are identical in mass and properties and a chemical reaction is a rearrangement of atoms. What we can conclude from his findings is that mass of a certain amount of an element/molecule/compound can be calculated because atoms of a given element are identical in mass and that all matter is conserved in a reaction.
The Avogadro's number is the number of atoms of an element that is weighs the relative atomic mass of the element in grams. The Avogadro number is named after Amedeo Avogadro, who in 1811 first proposed that the volume of a gas (at a given pressure and temperature) is proportional to the number of atoms or molecules regardless of the nature of the gas. The value of Avogadro's number is 6.022 × 1023 mol−1. Avogadro never actually calculated the number himself, but he did suggest, but Loschmidt was the first scientist, who we know of, to have suggested it, so the number, which it discoverer Jean Perrin proposed to be named after Avogadro, should be named after Loschmidt.
A modern definition of mole is:
“... as the amount of substances of a system that contains as many “elemental entities” (e.g. atoms, molecules. ions, electrons) as there are atoms in 12 g of carbon-12.
What this definition means is that as 6.022 × 1023 mol−1 of carbon-12 is 12 g, just as a mol of hydrogen-1 is 1 g and a mole of oxygen-16 is 16 g. A mole is defined as this because 1 Ar is defined as 1/12 of carbon-12 relative atomic mass.
So to summarise:
1 gram = 1 mole of amu
therefore
1 gram = 6.022 × 1023 mol−1 amu
