What Is the Difference Between Molar Mass & Molecular Weight?

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Molar mass and molecular weight are often confused, but their values are very different. Molar mass is the mass of one mole of a substance, while molecular weight is the mass of one molecule of a substance. One mole is the number of particles, such as atoms, molecules, ions or electrons, in a substance. The key to the difference between molar mass and molecular weight is the difference between a mole and a molecule.

Mole vs Molecule

A unit called the mole (sometimes called mol) is a convenient way of counting atoms in a substance. It allows scientists to predict the masses of different substances involved in chemical reactions. One mole is the Avogadro number of particles, atoms, molecules, ions or electrons in a substance. The Avogadro number is 6 x 10^23, i.e. 6 with 23 zeros after it. So 1 mole of oxygen is an amount of oxygen containing (6 x 10^23) number of oxygen molecules, and 1 mole of carbon is an amount of carbon containing (6 x 10^23) number of carbon atoms. A mole may contain particles of molecules, atoms or ions of a certain specific number.

A molecule is the smallest particle in an element or compound that possesses the chemical properties of that element or compound. Molecules are made up of atoms held together by chemical bonds.

Molar Mass

Often, people use the terms mass and weight interchangeably. Despite not being technically correct, it doesn't affect calculations. Molar mass is the mass of a mole of molecules, measured in grams or kilograms per mole. In other words, the molar mass of a compound tells you the mass of one mole of that substance.

For example, water has two hydrogen atoms and one oxygen atom. As per the periodic table, the atomic weight of hydrogen is 1 gram, and the atomic weight of oxygen is 16 grams. To calculate the molecular weight of one water molecule, you add (2 x 1) + 16 = 18 grams. The total molar mass of water is 18 grams per mole.

Molecular Weight

Molecular weight is the mass of one molecule, measured in atomic mass units (amu). For example, water has two hydrogen atoms and one oxygen atom. As above, the atomic weight of hydrogen is 1 gram, and the atomic weight of oxygen is 16 grams. To calculate the molecular weight of one water molecule, you add (2 x 1) + 16 = 18 grams. The total molecular weight of water is 18 grams.

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About the Author

Claire is a writer and editor with 18 years' experience. She writes about science and health for a range of digital publications, including Reader's Digest, HealthCentral, Vice and Zocdoc.

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