The mole links grams to atoms
Atoms are far too small and too many to count, so chemistry counts them in moles. One mole is 6.022 × 10²³ particles. To turn a weight into a number of moles, divide by the molar mass.
Here m is the mass in grams and M is the molar mass in grams per mole. Water's molar mass is 18 g/mol, so 36 grams of water is 2 moles.
Molar mass comes from the periodic table
Add up the atomic masses of every atom in the formula. Water (H₂O) is two hydrogens (1 each) plus one oxygen (16), giving 18 g/mol. Carbon dioxide (CO₂) is 12 + 2×16 = 44 g/mol.
Why chemists count in moles
Reactions happen atom-by-atom, in whole-number ratios. Working in moles keeps those ratios exact, so you know precisely how much of one substance reacts with another — something grams alone cannot tell you.
Frequently asked questions
How do you convert grams to moles?
Divide the mass in grams by the molar mass in grams per mole: n = m ÷ M. For water (M = 18 g/mol), 36 grams is 36 ÷ 18 = 2 moles.
What is a mole?
A mole is a fixed count of particles: 6.022 × 10²³ of them (Avogadro's number). It lets chemists weigh out a known number of atoms or molecules instead of counting them.
What is molar mass?
The mass of one mole of a substance, in grams per mole (g/mol). You get it by adding the atomic masses from the periodic table — water is 2×1 + 16 = 18 g/mol.
How many grams are in one mole?
Exactly the molar mass of the substance, expressed in grams. One mole of carbon (12 g/mol) is 12 grams; one mole of water is 18 grams.
What is Avogadro's number?
6.022 × 10²³ per mole — the number of particles in one mole. It is huge because atoms are tiny, so even a spoonful of water holds many moles' worth of molecules.