The
'mole'

**A mole is the Avogadro number of particles**..
rather like a dozen is twelve of whatever, the mole is just that, an Avogadro number of whatever - I quite fancy a mole of pennies (or even better pounds!)!

The chemists define the Avogadro Number in terms of carbon 12 atoms: the number of carbon-12 atoms in 12 grams (0.012 kg) of unbound carbon-12 in its rest-energy electronic state.

The **Avogadro number** is a big number **6.02
x 10 **^{23}

= 602 000 000 000 000 000 000 000

= six hundred and two thousand, million, million,
million!

It is given the symbol **N**_{A} (Number_{Avogadro})
by physicists and L (because in German scientific literature, sometimes also known as the Loschmidt constant) by chemists.

When we are dealing with atoms, having a large number
like this to compute with is valuable.

The link between mass and number of particles is
simple:

The mass of one particle (smallest unit - atom
or molecule)

expressed in grams contains N_{A} particles.

eg.

If you have 12g of carbon (atomic mass 12) there
are 6.02 x 10 ^{23} atoms of carbon in your sample. You have
one mole.

If you have 2g of hydrogen (atomic mass 1, but diatomic
molecules go round in pairs therefore the smallest unit has a molecular
mass of 2) there are 6.02 x 10 ^{23} molecules of hyrdogen in
your sample. You have one mole.

If you have 18g of water (atomic mass of hydrogen
is 1 and of oxygen is 16, therefore the molecular mass of H_{2}O
is 18) there are 6.02 x 10 ^{23} molecules of water in your
sample. You have one mole.

**Try the following questions:**

1. How many atoms of lead are found in a 2mg sample
? (atomic mass of lead is 210g) ? Answer

2. What would be the mass of one hundred million million
(100 000 000 000 000 = 10^{14}) atoms of Uranium 238 (atomic
mass is 238)?
Answer