Moles and molar mass

Atoms are far too small to count one by one, so chemists count them in batches called moles. One mole is 6.022 × 10²³ things (Avogadro's number), the same way a dozen is 12. A mole of water is 6.022 × 10²³ water molecules.

The number was chosen so that a mole of an element weighs its atomic mass in grams. One mole of carbon-12 is 12 g; a mole of oxygen atoms is about 16 g. That weight per mole is the molar mass, in grams per mole (g/mol). For a compound, add up its atoms: water, H₂O, is 2 × 1 + 16 = 18 g/mol.

In this course's practice, use these rounded atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, S = 32, Cl = 35.5, Ca = 40. A periodic table gives more digits (H is 1.008, so water is really 18.02), and real lab work uses them.

Moles = mass ÷ molar mass. 36 g of water is 36 ÷ 18 = 2 moles. Going the other way, mass = moles × molar mass: 0.5 mol of carbon dioxide (12 + 2 × 16 = 44 g/mol) is 22 g. And molecules = moles × 6.022 × 10²³.

Percent composition says how much of a compound's mass comes from each element. In water, 16 of every 18 g is oxygen: 16 ÷ 18 = 88.9% oxygen by mass, even though only one atom in three is oxygen.