Molecular Weight

Last updated: July 15, 2018

What Does Molecular Weight Mean?

Molecular weight is the unit of measurement used to describe the mass of a molecule. Molecular weight is determined by combining the atomic weights of each atom comprising the molecule.

Each element has a distinct atomic weight that represents the average of the atomic mass for that element. An average is calculated because each element has distinct isotopes each with a slightly different atomic weight.

A mass spectrometry device is often used to measure the mass of a substance or molecule.

Molecular weight is also referred to as formula mass, relative molecular mass or molecular mass.


WorkplaceTesting Explains Molecular Weight

The molecular weight of a substance can be calculated by combining the atomic weight of the individual elements that form the molecule as identified by its chemical formula. For example, the molecular mass of salt is 58.4 grams or the combined average atomic weight of one atom of sodium and one of chloride.

While they are sometimes thought to be interchangeable, molecular weight is not the same as molar weight. Molar weight refers to the mass of one mole of a substance. A substance may include a mix of different molecules, atoms, and other distinct materials. Whereas, a molecule represents a single substance made from one or more elements chemically combined. When a substance contains only one type of molecule, the molar mass and molecular weight should be the same.

However, if two types of molecules are combined, the molar mass may differ from the molecular weight of the individual molecules. For example, the molecular mass of NaCl is 58.4 grams/mol and the molecular mass of water is approximately 18 grams/mol. However, the molecular weight of a mole of saltwater will vary depending upon the ratio of salt to water in the solution.



Relative Molecular Mass, Molecular Mass, Formula Mass

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