When small amounts of substances are being used in chemistry very large numbers of particles are involved.

One teaspoon of copper sulfate… Contains 3.0 x Cu 2+ and SO 4 2- ions. 30,000,000,000,000,000,000,000 ions.

One cup of water… Contains 5.30 x water molecules 5,300,000,000,000,000,000,000,000 water molecules

A 500mL conical flask… Contains 2.40 x oxygen molecules 2,400,000,000,000,000,000,000 O 2 molecules

When small amounts like this are being used, how do people know the amount of particles?

Do people count out everything? How does: A builder count 5536 nails to complete a house? An office worker count 2400 sheets of A4 paper? A banker count a bag of $2 coins? Somebody buy eggs or socks? Somebody count the amount of cards in two packs?

Chemist need a convenient method to count the number of particles (atoms, molecules, ions etc). Because atoms are so small and there are so many of them in even the smallest sample, it is impossible to count them directly. So chemists created the counting unit called the mole.

The mole, symbol mol is the basic SI unit used to measure the chemical amount of a substance. The mole is defined: “as the number of atoms present in exactly 12 g of the carbon-12 isotope”

6.02 x ,000,000,000,000,000,000,000 And… there are atoms in 12 g of the carbon-12 isotope

1 mol = 6.02 x particles So…

So a mole of anything contains 6.02 x particles and this number has a special name called Avogadro’s number (N A ) N A = 6.02 x particles = 1 mol

Example..

Is so large that… 1 mol of marbles would cover the surface of the Earth to a depth of more than 6km.