The Mole
In chemistry we do not think of the mole as a tiny furry mammal, we think of the mole as a fixed number of particles. In the same way as a dozen is the number 12 and a couple is the number 2, the mole is the number of atoms in 12g of a sample of carbon. You do not need to know how many atoms this is until you join the higher class, but you should know that this number runs into many billions. If you remember from Topic 3 one carbon atom has a mass of 12 amu but a mole of carbon (containing this very large number of atoms) has a mass of 12 grams. Put this way the mole can be thought of as the conversion factor to turn amu into grams. This is why many refer to the mole as the formula mass expressed in grams or
the gram formula mass (gfm).
Example 1
Q What is the mass of 1 mole of sodium ?
A The formula for sodium is Na. if we look up page 4 of the data booklet we find that
1 sodium atom has a mass of 23 amu
1 mole of sodium would contain enough atoms to make a mass of 23g
so the mass of 1 mole of sodium or gfm is 23g
Example 2
Q What is the mass of 1 mole of nitrogen ?
A The formula of nitrogen is N
2
(this means that there are 2 x N atoms) so the formula mass = 2 x 14 = 28 amu
so the gfm = 28g
So the mass of 1 mole of nitrogen is 28g
Example 3
Q What is the mass of 2 mole of calcium carbonate ?
A The formula of calcium carbonate is Ca CO
3 so the formula mass is 1 x 40 = 40
1 x 12 = 12
3 x 16 = 48
Total = 100 amu
so the gfm is 100g
so 2 moles of calcium carbonate is 2 x 100g = 200g

Ready for more complex calculations?
The calculations can become more complex than this but are easily carried out if you learn these two formula triangles.
Triangle 1 m m
Triangle 2 n n
n X gfm where m = mass of substance
n = number of moles
gfm = gram formula mass
To use this triangle you must remember to convert your mass into grams
This allows us to create three equations : m = n x gfm n = m / gfm gfm = m / n
c c = n / v v = n / c x v where n = number of moles
c = concentration in mol /l or mol l –1
v = volume
To use this triangle you must remember to convert your volume into litres
This allows us to create three equations : n = c x v
Example 1
Q How many moles of sodium sulphate are their in 426g of the substance?
A Formula of sodium sulphate is Na
2
SO
4
2 x 23 = 46
1 x 32 = 32
4 x 16 = 64
Total = 142
formula mass = 142 amu so gfm = 142g using the formula n = m n = 426 = 3 moles
gfm 142
so the number of moles present 426g is 3 moles.

Example 2
Q What volume of 0.1 mol l –1 sodium hydroxide solution is needed to make a solution containing 0.2 moles ?
A using the equation v = n v = 0.2 = 2 litres
c 0.1
Example 3
Q What is the concentration of a solution of sodium chloride which has 2 moles dissolved in 500 ml of water.
A using the equation c = n c = 2 = 4 mol l
Example 4
v 0.5
–1
Q What is the concentration of a solution of ammonium carbonate which has
192g dissolved in 250 ml of water.
A we will want to use the equation c = n/v and to do this we will need to first of all find out n. To do this we will use the equation n = m/gfm gfm (NH
4
)
2
CO
3
2 x 14 = 28 so n = 192/ 96 = 2 moles
8 x 1 = 8 v = 250 ml = 0.25 l
1 x 12 = 12
3 x 16 = 48
Total = 96g
so c = 2/0.25 = 8 mol l –1