The number in a mole is called Avogadro’s number : 6.022142 x 10 23, after the 19th-century Italian scientist who first proposed how to measure the number of molecules in a gas. Just as 1 mol of atoms contains 6.022 × 10 23 atoms, 1 mol of eggs contains 6.022 × 10 23 eggs. According to the most recent experimental measurements, this mass of carbon-12 contains 6.022142 × 10 23 atoms, but for most purposes 6.022 × 10 23 provides an adequate number of significant figures.
The mole is used for this purpose.Ī mole is defined as the amount of a substance that contains the number of carbon atoms in exactly 12 g of isotopically pure carbon-12. Any readily measurable mass of an element or compound contains an extraordinarily large number of atoms, molecules, or ions, so an extraordinarily large numerical unit is needed to count them. Atoms are so small, however, that even 500 atoms are too small to see or measure by most common techniques. Sheets of printer paper are packaged in reams of 500, a seemingly large number. 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.
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: This site explains how to find molar mass.\right ) \right ] \) The reason is that the molar mass of the substance affects the conversion. To complete this calculation, you have to know what substance you are trying to convert. Using the chemical formula of the compound and the periodic table of elements, we can add up the atomic weights and calculate molecular weight of the substance.Ī common request on this site is to convert grams to moles. These relative weights computed from the chemical equation are sometimes called equation weights. For bulk stoichiometric calculations, we are usually determining molar mass, which may also be called standard atomic weight or average atomic mass.įormula weights are especially useful in determining the relative weights of reagents and products in a chemical reaction. This is not the same as molecular mass, which is the mass of a single molecule of well-defined isotopes. This is how to calculate molar mass (average molecular weight), which is based on isotropically weighted averages. The atomic weights used on this site come from NIST, the National Institute of Standards and Technology. The formula weight is simply the weight in atomic mass units of all the atoms in a given formula. When calculating molecular weight of a chemical compound, it tells us how many grams are in one mole of that substance. The percentage by weight of any atom or group of atoms in a compound can be computed by dividing the total weight of the atom (or group of atoms) in the formula by the formula weight and multiplying by 100.įinding molar mass starts with units of grams per mole (g/mol). If the formula used in calculating molar mass is the molecular formula, the formula weight computed is the molecular weight. In chemistry, the formula weight is a quantity computed by multiplying the atomic weight (in atomic mass units) of each element in a chemical formula by the number of atoms of that element present in the formula, then adding all of these products together.