For example, cans of soda come in a six-pack, eggs are sold by the dozen (12), and pencils often come in a gross (12 dozen, or 144). Many familiar items are sold in numerical quantities that have unusual names. The quantity of a substance that contains the same number of units (e.g., atoms or molecules) as the number of carbon atoms in exactly 12 g of isotopically pure carbon-12., from the Latin moles, meaning “pile” or “heap” ( not from the small subterranean animal!). The unit that provides this link is the mole (mol). To analyze the transformations that occur between individual atoms or molecules in a chemical reaction it is therefore absolutely essential for chemists to know how many atoms or molecules are contained in a measurable quantity in the laboratory-a given mass of sample. In the laboratory, for example, the masses of compounds and elements used by chemists typically range from milligrams to grams, while in industry, chemicals are bought and sold in kilograms and tons.
Because the masses of individual atoms are so minuscule (on the order of 10 −23 g/atom), chemists do not measure the mass of individual atoms or molecules. The problem for Dalton and other early chemists was to discover the quantitative relationship between the number of atoms in a chemical substance and its mass.
NACL MOLAR MASS SERIES
We also described the law of multiple proportions, which states that the ratios of the masses of elements that form a series of compounds are small whole numbers. In Dalton’s theory each chemical compound has a particular combination of atoms and that the ratios of the numbers of atoms of the elements present are usually small whole numbers. The same calculation can also be done in a tabular format, which is especially helpful for more complex molecules:
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