Chemical Quantities

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

One teaspoon of copper sulfate…

Chemical Quantities

Contains 3.0 x 1022 Cu2+ and SO4

2- ions.

30,000,000,000,000,000,000,000 ions.

One cup of water…

Chemical Quantities

Contains 5.30 x 1024 water molecules

5,300,000,000,000,000,000,000,000 water molecules

A 500mL conical flask…

Chemical Quantities

Contains 2.40 x 1021 oxygen molecules

2,400,000,000,000,000,000,000 O2 molecules

Chemical Quantities

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?

Chemical QuantitiesChemist 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 MoleThe 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 1023

602,000,000,000,000,000,000,000

The MoleAnd… there are

atoms in 12 g of the carbon-12 isotope

1 mol = 6.02 x 1023 particles

The MoleSo…

The MoleSo a mole of anything contains 6.02 x 1023 particles and this number has a special name called Avogadro’s number (NA)

NA = 6.02 x 1023 particles = 1 mol

Example..

• 1 mol of carbon contains 6.02 x 1023 carbon atoms.

• 0.50 mol of CO2 contains () = 3.00 x 1023 CO2 molecules.

• 1 mol of methane, CH4, contains 6.02 x 1023 carbon atoms and (4 x 6.02 x 1023) = 2.41 x 1024 hydrogen atoms.

Mole Examples

Avogadro’s Number, NA

Is so large that…

1 mol of marbles would cover the surface of the Earth to a depth of more than 6km.

Avogadro’s Number, NA