After the redefinition of units, this relationship is only nearly equivalent, although the difference is negligible for all practical purposes. Prior to the 2019 redefinition of SI base units quantities expressed in daltons (Da) were by definition numerically equivalent to molar mass expressed in the units g/mol and were thus strictly numerically interchangeable. The molecular mass is more commonly used when referring to the mass of a single or specific well-defined molecule and less commonly than molecular weight when referring to a weighted average of a sample. The terms "molecular mass", "molecular weight", and "molar mass" may be used interchangeably in less formal contexts where unit- and quantity-correctness is not needed. In molecular biology, the mass of macromolecules is referred to as their molecular weight and is expressed in kDa, although the numerical value is often approximate and representative of an average. When the molecular weight is given with the unit Da, it is frequently as a weighted average similar to the molar mass but with different units. The definition of molecular weight is most authoritatively synonymous with relative molecular mass however, in common practice, use of this terminology is highly variable. The molar mass is usually the more appropriate quantity when dealing with macroscopic (weigh-able) quantities of a substance. That makes the molar mass an average of many particles or molecules, and the molecular mass the mass of one specific particle or molecule. The molar mass is defined as the mass of a given substance divided by the amount of a substance, is expressed in grams per mol (g/mol). The molecular mass and relative molecular mass are distinct from but related to the molar mass. The derived quantity relative molecular mass is the unitless ratio of the mass of a molecule to the atomic mass constant (which is equal to one dalton). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. For bulk stoichiometric calculations, we are usually determining molar mass, which may also be called standard atomic weight or average atomic mass.The molecular mass ( m) is the mass of a given molecule. 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. To complete this calculation, you have to know what substance you are trying to convert. 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.Ī common request on this site is to convert grams to moles. If the formula used in calculating molar mass is the molecular formula, the formula weight computed is the molecular weight. 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. These relative weights computed from the chemical equation are sometimes called equation weights. The formula weight is simply the weight in atomic mass units of all the atoms in a given formula.įormula weights are especially useful in determining the relative weights of reagents and products in a chemical reaction. When calculating molecular weight of a chemical compound, it tells us how many grams are in one mole of that substance. 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.įinding molar mass starts with units of grams per mole (g/mol).
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