Molar MassWhat is molar mass?
Molar mass is the mass of one mole of a substance. It is written with the letter M and measured in grams per mole, g/mol. Water has a molar mass of about 18.02 g/mol. That means one mole of water, a fixed and very large number of water molecules, has a mass of about 18.02 grams.
Why chemists need it
Atoms and molecules react in whole numbers. Two hydrogen molecules join one oxygen molecule to make two molecules of water. But nobody can count out molecules: they are far too small and far too many. What a chemist can do is weigh. Molar mass is the bridge between the two. It tells you how many grams to weigh so that you get the number of particles you want.
A mole is simply a counting unit, like a dozen, but very much bigger. One mole contains exactly 6.02214076 × 1023 particles. This number is called the Avogadro constant. So if you weigh out 18.02 g of water you have that many water molecules, and if you weigh out 58.44 g of common salt you have the same number of formula units of sodium chloride.
Where the number comes from
Every element has an atomic weight, printed in the periodic table. Carbon is 12.011, hydrogen 1.0080, oxygen 15.999. The molar mass of an element in grams per mole has the same number as its atomic weight. For a compound you add the atomic weights of all the atoms in its formula.
Take carbon dioxide, CO2. It has one carbon atom and two oxygen atoms:
12.011 + 2 × 15.999 = 44.009 g/mol
So 1 mole of carbon dioxide has a mass of about 44.01 g.
Three things molar mass lets you do
- Turn grams into moles. Divide the mass by the molar mass. 88 g of carbon dioxide is 88 ÷ 44.01 = about 2 moles.
- Turn moles into grams. Multiply. Half a mole of sodium chloride is 0.5 × 58.44 = 29.22 g.
- Find how much of each element is in a compound. In water, oxygen gives 15.999 of the 18.015, which is 88.8 percent of the mass.
Molar mass is not the same for every sample size
A common worry is whether a bigger sample has a bigger molar mass. It does not. Molar mass belongs to the substance, not to the sample. A drop of water and a bucket of water both have a molar mass of 18.02 g/mol. The bucket simply holds more moles.
A note on the words
Older books say molecular weight or formula weight. School books in India often say molecular mass or gram molecular mass. In everyday sums they all lead to the same number. The small differences in meaning are set out on the page about molar mass and molecular mass.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.