Calculation of Empirical Formulae.
Example
1. A compound contains 75% carbon and 25% hydrogen. What is its empirical
formula?
Elements
Amount in question (% or mass)
Atomic mass (periodic table)
Number of moles = Amount in question
Atomic mass
Carbon
75
12
75
12
=6.25
Mole ratio (divide each number of moles by
the smallest number of moles)
6.25
6.25
=1
25
6.25
=4
C H4
Empirical formula
2. An oxide of carbon contains 27% carbon.
What is its empirical formula?
Elements
Amount in question (% or mass)
Atomic mass (periodic table)
Number of moles = Amount in question
Atomic mass
Mole ratio (divide each number of moles by
the smallest number of moles)
Hydrogen
25
1
25
1
=25
Carbon
27
12
27
12
= 2.25
=2.25
2.25
=1
Oxygen
2.25
=
Empirical formula
3. Fluorspar is made of calcium and fluorine. If 51% is calcium, calculate the
empirical formula.
Calcium fluorine
Elements
Amount in question (% or mass)
Atomic mass (periodic table)
Number of moles = Amount in question
Atomic mass
Mole ratio (divide each number of moles by
the smallest number of moles)
Empirical formula
Empirical
formula
CH4
4. 2.70g of aluminium is combined with 10.65g of chlorine. What is the empirical
formula of the new substance?
Elements
Amount in question (% or mass)
Atomic mass (periodic table)
Number of moles = Amount in question
Atomic mass
Mole ratio (divide each number of moles by
the smallest number of moles)
Empirical formula
5. 1.68g of iron is combined with 0.48g of oxygen. What is the empirical formula of
the new compound ?
Elements
Amount in question (% or mass)
Atomic mass (periodic table)
Number of moles = Amount in question
Atomic mass
Mole ratio (divide each number of moles by
the smallest number of moles)
Empirical formula
6. Saltpetre is a potassium salt. It is 13.9% nitrogen ,38.6% potassium and 47.5%
oxygen. What is its empirical formula? (Hint- the table needs an extra column but
you work out everything else in the same way)
Elements
Amount in question (% or mass)
Atomic mass (periodic table)
Number of moles = Amount in question
Atomic mass
Mole ratio (divide each number of moles by
the smallest number of moles)
Empirical formula
Extension- This one has an extra twist to it!.
7.A hydrocarbon has 80% carbon and 20 %hydrogen.
a) Calculate its empirical formula.
b) When the mass of a single molecule of this compound was analysed, it was
found to have a mass of 30g. What must the true (molecular) formula of this
substance be?
Calculation of Reacting Masses.
Example 1
Calculate the mass of iron sulphide produced when 5.6g of iron reacts
completely with excess sulphur.
Equation
Fe + S
FeS
This means;
1mole of Fe reacts with 1mole of S to make 1mole of FeS
How many
moles are we
using?
How many
moles of
product will
be made ?
Amount of
product
produced.
We are using 5.6g Fe.
Number of moles of iron being used is 5.6 = 0.1 Moles
56
Since 1mole of Fe reacts with 1mole of S to make 1mole of FeS
Then 0.1 moles of Fe will produce 0.1moles of FeS
1 mole of FeS = (56+32) = 88g
So, 0.1 mole of FeS will have a mass of 0.1x 88 =8.8g
Example 2
How much carbon would be needed to make 8.8g of carbon dioxide?
Equation
C + O2
CO2
This means;
__mole of C makes 1mole of CO2
How many
moles are
we making?
We have made __g of CO2
1 mole of CO2 has a mass of _________g
Number of moles of CO2 that have been made =Mass of CO2 made
Mass of 1mole of CO2
=
How many
moles of
product will
be made ?
Amount of
reactant
used
Since 1mole of CO2 is made from 1mole of C,
This means we must have used ______ moles of C.
Mass of carbon =Relative atomic mass of carbon x number of moles
used
=
=
3.Calcium carbonate (CaCO3) decomposes when heated to give calcium oxide
(CaO)and carbon dioxide (CO2)
a) Write a balanced equation for the reaction.
b)Calculate the mass of calcium carbonate needed to make 560g of calcium oxide.
4. Copper carbonate decomposes to give copper oxide and carbon dioxide.
a) Write a balanced equation for the reaction.
b)What mass of copper carbonate is needed to produce 8g of copper oxide?
5.How much calcium oxide is produced by heating 25 tonnes of calcium carbonate?
6.What mass of water is produced by completely burning 15kg of butane?
2C4H10 + 13O2
8CO2
+ 10H2O.