Chemistry: Molar Mass

Molar Mass

When speaking about doing chemical reactions, it isn't very handy to say that you want "one mole of compound X to react with two moles of compound Y." That may be what you need to do, but unfortunately, there are no machines that can count 6.02 × 1023 molecules.

Molecular Meanings

The molar mass of a substance is the weight of 6.02 × 1023 atoms or molecules of that material in grams. The unit of molar mass is given as grams/mole, usually abbreviated as g/mol.

As a result, we're going to have to learn to find something called the molar mass—the weight of one mole of a chemical compound. That way, if somebody says that we need to use two moles of water in a reaction, we can go to a balance and just weigh it instead of having to individually count a very large number of molecules.


Other common terms that mean the same thing as "molar mass" are "molecular mass," "molecular weight," and even "gram formula mass."

Finding the molar masses of compounds isn't difficult if you know their formulas. To do this, multiply the numbers of atoms of each element in a compound by their atomic masses from the periodic table. When you add these numbers up, you come up with the molar mass of the compound.

For example, let's find the molar mass of sulfuric acid, H2SO4:

Element Number of Atoms Atomic Mass (g) Mass × Atoms
H 2 1.01 2.02
S 1 32.07 32.07
O 4 16.00 64.00
Total: 98.09 g

As a result, we would say that the molar mass of sulfuric acid is "98.09 g/mol."

You've Got Problems

Problem 1: Find the molar masses of the following compounds:
(a) Na2SO4
(b) nitrogen trichloride
(c) fluorine

Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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