These relative weights computed from the chemical equation are sometimes called equation weights. 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.įormula weights are especially useful in determining the relative weights of reagents and products in a chemical reaction. 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. For bulk stoichiometric calculations, we are usually determining molar mass, which may also be called standard atomic weight or average atomic mass. 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. This site explains how to find molar mass. The reason is that the molar mass of the substance affects the conversion. It is located in group seventeen, period three and block p of the periodic table. Chlorine has seventeen protons and eighteen neutrons in its nucleus, and seventeen electrons in three shells. It has an atomic weight of 35.450 and a mass number of 35. To complete this calculation, you have to know what substance you are trying to convert. Chlorine is the 17th element in the periodic table and has a symbol of Cl and atomic number of 17. 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.Ī common request on this site is to convert grams to moles.
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