Moles and molar mass — Chemistry, 14–17 years
The mole is chemistry’s way to count enormous numbers of particles by weighing them. Chemistry, 14–17 years.
A mole is a chemist’s counting packet
Atoms and molecules are far too small to count one by one. A mole is a fixed packet of 6.022 × 10²³ particles, just as a dozen means 12 objects. Molar mass tells us how many grams one mole of a substance weighs, so a balance can tell us how many particles we have.
Why chemists needed the mole
A reaction recipe may require particles in exact proportions, but a laboratory cannot scoop out individual atoms. Chemists therefore needed a bridge between the microscopic world and grams that can be measured. The mole supplies that bridge: its fixed size connects particle ratios in an equation with real quantities on a balance.
From grams to particles
Suppose we have 117.0 g of sodium chloride, NaCl. First, its molar mass is 23.0 + 35.5 = 58.5 g/mol. Next, divide the sample mass by the molar mass: 117.0 ÷ 58.5 = 2.00 mol. Finally, multiply by Avogadro’s number: 2.00 × 6.022 × 10²³ = 1.204 × 10²⁴ formula units.
Grams are not moles
A common mistake is to treat 10 g of every substance as 10 mol. That feels reasonable because grams measure how much we have, but different particles have different masses. Ten grams of hydrogen contains many more particles than 10 g of copper, so always use the substance’s own molar mass.
Moles in the real world
Moles help a factory make the right amount of fertiliser, medicine or plastic from measured ingredients. They also let a pharmacist compare a dose of one substance with a dose of another, even when their particles have different masses. The idea is useful whenever invisible particles must be controlled through visible amounts.
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