Molar mass

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Molar mass is the mass of one mole of a chemical element or chemical compound.^[1] It is commonly used in stoichiometric calculations of bulk substances in chemistry. Its primary purpose is as a conversion factor between the number of grams of a pure substance, which can be measured directly, and the number of moles of that substance, which has greater chemical significance. This allows for using the appropriate number of molecules of a substance regardless of the mass. For example, if an equal number of molecules of two substances are needed for a reaction but the molar mass of one substance is twice that of the other, twice as many grams will be needed of that substance to give the same number of molecules.

A mole is defined as Avogadro's number, approximately 6.022x10²³, of particles of any kind of substance (atoms, ions, molecules, or formula units).

In chemistry, the unit of molar mass is g/mol. The atomic mass listed for the elements on the periodic table can be interpreted in two ways: first as the mass of a single average atom of the element in unified atomic mass units (u), or secondly as the mass of one mole of the element in grams. This second use is the molar mass of the element and is most useful in stoichiometric calculations. In physics, molar mass is usually defined in kilograms per kilomole (kg/kmol). Molar mass is different from Molecular mass which is the mass of one molecule.

In linear polymers not every polymer chain consists of the same amount of repeating units. A given polymer sample is said to be made up of a mixture of macromolecules with a certain molar mass distribution.^[2]

1 Example
2 Molar mass versus molecular mass
3 References
4 See also
5 External links

[edit] Example

Let us see approximately how many grams are in 2.3 moles of table sugar with a chemical formula of C₁₂H₂₂O₁₁. The atomic masses of carbon, hydrogen, and oxygen are 12.0, 1.0, and 16.0 g/mol respectively. Thus the molar mass of sucrose is its sum: (12.0 * 12 + 1.0 * 22 + 16.0 * 11) = 342.0 g/mol. The mass of 2.3 moles of sugar is then 2.3 mol * 342.0 g/mol = 786.6 g.

[edit] Molar mass versus molecular mass

The molar mass and molecular mass are approximately equal; however, they differ in their units, g/mol versus u, and are often subtly different numerically. The molar mass is almost always computed using isotopically weighted averages whereas, technically the molecular mass is the mass of a single molecule. The isotopically weighted averages are what is usually found on a periodic table and is usually a number recommended by IUPAC that represents the most likely weight of a mole of the substance found in a laboratory. This takes into account the natural abundance of, usually heavier, isotopes as well as the variation in their natural abundance in different places on the earth. Since a molecular mass is the mass of a single molecule, these numbers are mostly different to some fraction. Additionally the confidence, or number of significant figures after the decimal, is different. It is common, even amongst professional chemists, to use the terms interchangeably since for most common applications the difference is insignificant. This can, however, on occasion lead to substantive confusion. Due to this common practice some areas of chemistry have developed their own more specific terms such as monoisotopic mass and average mass.

Due to these subtle differences and the inherent nature of the molar mass it is always more correct, accurate and consistent to use molar mass in any bulk stoichiometric calculations.