4.4.3 Mole ratios in formulae

Question 1

What is the empirical formula?

Answer 1

The empirical formula of a substance contains the simplest mole ratio of its elements.

Question 2

How is the empirical formula calculated?

Answer 2

1. Convert the masses (or percentages) into moles.
2. Cancel down the mole ratio.
3. Write the simplest mole ratio into a formula.
   USE MASS MOLES RATIO TABLE

Question 3

What is the molecular formula?

Answer 3

The molecular formula of a substance contains the actual numbers of atoms in a molecule.

Question 4

How do you calculate molecular formula?

Answer 4

You calculate the molecular formula using the empirical formula and the RFM.
e.g. Molecular formula of CH2 with RFM of 70
RFM of CH2 = 14
70/14 = 5
molecular formula = 5 x CH2 = C5H10

Question 5

What is water of crystallisation?

Answer 5

Hydrated salt crystals contain water trapped inside them in a fixed mole ratio. This water is called water of crystallisation, and it is written like this:
CuSO4 . 5H2O
In the hydrated copper sulphate crystals above, the mole ratio of salt:water is 1:5.
The mole ratio is always in the from 1:x

Question 6

What happens to hydrated salt crystals when they are heated?

Answer 6

They lose their water.
Hydrated salt –> Anhydrous salt + Water
The water was once the water of crystallisation.

Question 7

How do you calculate water of crystalliation?

Answer 7

Water of crystallisation is calculated from the masses of slat and water produced on heating.
e.g. Calculating x if 16.7g of MgCl2.xH2O crystals decreased in mass to 9.5g on heating.
1. Calculate the mass of water lost:
mass of water lost = 16.7g - 9.5g = 7.2g
2. Draw a mass, moles, ratio table:
MgCl2 H2O
mass 9.5 7.2
moles 0.1 0.4
ratio 1 4

therefore x = 4