1. Empirical Formulae
The empirical formula of a compound is the simplest ratio of the different atoms in it.
For example, for ethane (C2H6)it is CH3.
You can work out the empirical formula of a compound if you know the mass of each element in it.
This is really the reverse of finding the % mass of a compound from its formula.

2. Step 1 Find the relative atomic masses of the elements involved.
Example ;
What is the empirical formula of a compound that contains 27.3g carbon and 72.7g oxygen?
Step 1 Find the relative atomic masses of the elements involved.
Ar(C) = 12 and Ar(O) = 16
Step 2 Work out how many moles of each element this must be using; number of moles = mass Ar
carbon = 27.3/12 = oxygen = 72.7/16 = 4.55
Step 3 Divide both numbers of moles by the smallest number (2.275 in this case).
carbon = 2.275/2.275 = 1 oxygen = 4.55/2.275 = 2
In the empirical formula, there are 1 carbon atom and 2 oxygen atoms, so it is CO2.

3. How much:
copper oxide forms when 16 g of copper reacts with 4 g of oxygen?
chlorine is needed if 23 g of sodium makes 58.5 g of sodium chloride?
oxygen is needed if 20 g of hydrogen makes 180 g of water?
carbon is needed if 6.4 g of oxygen makes 8.8 g of carbon dioxide?
zinc is needed if g of chlorine makes 34 g of zinc chloride?

4. C2H8O2N2 CH3COOH Cl2C2H6 CH3COOC2H5
2. What is the empirical formula of a compound with these molecular formulae?
C4H8
C2H8O2N2
CH3COOH
Cl2C2H6
CH3COOC2H5

5. [H = 1, C = 12, N = 14, Al = 27, O = 16, Mg = 24, Cl = 35.5, I = 127]
3. What is the empirical formula of the following compounds containing:
[H = 1, C = 12, N = 14, Al = 27, O = 16, Mg = 24, Cl = 35.5, I = 127]
2 g hydrogen and 16 g oxygen.
0.05 g hydrogen and 0.8 g oxygen.
0.6 g carbon and 0.8 g oxygen.
0.6 g carbon and 1.6 g oxygen.
1.2 g magnesium and 3.55 g chlorine
5.4 g of aluminium and 76.2 g iodine

6. [H =1, C = 12, Ca = 40, Cl = 35.5, N = 14, Na = 23, O = 16, S = 32]
4. Calculate the empirical formula for these compounds from mass:
[H =1, C = 12, Ca = 40, Cl = 35.5, N = 14, Na = 23, O = 16, S = 32]
A compound containing 27 g of calcium, 21.6 g of oxygen and 1.35 g of hydrogen
A compound containing 36.5 g of sodium, 25.4 g sulphur and 38.1 g oxygen
A compound contains 14.2 g of chlorine, 4.8 g carbon and 0.8 g of hydrogen. It has a molar mass of 99 g/mol.
What is the empirical formula?
What is its molecular formula?