Moles and Avagadro's Number

Atoms and molecules are extremely small and weighing or counting them directly isn't possible. This is where the mole comes in, giving chemists a way of relating atomic weights to real-world weights and measures, and working out the actual formulae of compounds. The mole is the bridge between the microscopic and macroscopic worlds.

Counting by Weighing

A mole is an amount of a substance that contains as many particles as there are atoms in exactly 12g of the isotope carbon-12. The particles can be anything - atoms, molecules, ions, electrons - but they must be specified. The number of atoms in 12g of carbon-12 is known as the Avogadro number, named for the 19th century scientist who originated the concept of the mole. it is 6.0221367 x 10^23, which is 602 billion trillion, or 602 with 21 zeros after it.

Moles allow us to count by weighing. The mole gives the atomic weight of an element in grams - quantities that chemists can work with in the real world. The mole isn't simple atomic weight expressed in grams; it can also be the molecular, or formula, weight expressed in grams. "Formula weight" is the sum of the atomic weights of atoms in a compound.

For instance, the atomic masses of the atoms that make up a molecule of water (H2O) are 1 atomic mass unit (amu) each for the 2 hydrogen atoms and 16 amu for the oxygen, giving a total of 18 amu. A mole of water therefore weighs 18g (the atomic weights of these elements are not quite round numbers because of isotopes, but have been rounded up here for simplicity.)

Moles in Action

The mole is a powerful tool for chemists. Say you have 22.99g of sodium (Na) and you want to make table salt by combining it with chlorine (Cl), but you don't want to waste reactants by having too much of one or the other. How do you know how much chlorine to use?

Handily the atomic weight of sodium is 22,99 amu, so you know you've got 1 mole of sodium. Given that you also know the formula of table salt is NaCl, you know that you need one atom of chlorine for each atom of sodium - or q mole of chlorine for each mole of sodium. The atomic weight of chlorine is 35,453, therefore you need 35,453g of chlorine.

In practice, no reaction is 100% efficient so not every particle of reactant will react, but you get the idea. Weighing gases is time-consuming, but helpfully its possible to convert the mole concept into measures of liguid and gaseous volume.

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