Chemical Formula Calculation
Revision Notes
Valency or combining power
 The valency of an element is the number of electrons lost, gained or
shared to from compounds. The valency of an element depends on the
number of valence electrons.
 In covalent molecules; the valency gives the number of covalent
bonds which the atoms can form.
 In ionic compounds; the valency gives the charge on the ions of
the element.
Relative Molecular Mass, Mr
The relative molecular mass of a molecule is the average mass of one
molecule of that element or compound compared to 1/12 of the mass of
one carbon-12 atom.
Relative Formula Mass, Mr
The relative formula mass of an ionic compound is the average mass of one
unit of that ionic compound compared to 1/12 of the mass of one carbon-12
atom.
Molar Mass
Molar mass refers to the mass of one mole of a substance (which could be
an element or a compound).
Example:
1. The relative atomic mass of magnesium is 24. This means that one mole of
magnesium atoms has a mass of 24 g. The molar mass of magnesium will
thus be 24 g/mol
Moles
A ‘mole’ simply refers to the amount of a certain substance.
Avagadro's number = 6.02 x 1023
Moles, concentration and volume
We can calculate the moles of a solution when we know the
concentration and volume.
Calculation Mole:
Molar volume of a gas
One mole of a gas always takes up the same volume at room temperature
Percentage yield
and pressure. That volume is 24 dm3 or 24,000 cm3.
Percentage yield describes how much reactant was successfully
converted into product.
Percentage purity
Percentage purity of a substance can be calculated by dividing the mass
of the pure chemical by the total mass of the sample, and then
multiplying this number by 100.
Empirical formula and molecular formula
The molecular formula is the actual number of atoms of each element in
a compound.
The empirical formula is the simplest whole number ratio of atoms of
each element in a compound.
Chemical equations and calculations test questions
1)A student had calculated the theoretical yield of a product was 8 g.
The actual yield obtained was 6 g. What was the percentage yield?
 48%
 75%
 133%
2. Which equation shows a substance dissolved in water reacting to
form a gas?
 Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
 C(s) + H2O(g) → CO(g) + H2(g)
 HCl(g) + NaOH(aq) → NaCl(aq) + H2O(l)
3) In this unbalanced symbol equation: Na + Cl2 → NaCl what is the
correct balanced equation?
 2Na + Cl2 → NaCl
 2Na + Cl2 → 2NaCl
 Na + Cl2 → 2NaCl
4) What is meant by the relative atomic mass of an element (also
represented as the symbol Ar)?
 It is the mass of an atom of a particular element measured in
grams
 It is the actual mass of an atom of an element compared to the
mass of a molecule of the element
 It is the average mass of the atoms of an element compared to the
mass of an atom of carbon-12
5) What is the relative atomic mass (Ar) of aluminium? Use the periodic
table to help you.
 13
 14
 27
6) What is the relative formula mass (Mr) of magnesium oxide (formula
MgO)? Use the periodic table to help you.
 40
 30
 20
7) What is the relative formula mass (Mr) for sulfuric acid (formula
H2SO4)? Use the periodic table to help you.
 50
 98
 49
8) What does the law of conservation of mass state?
 That the total mass of the reactants will always be the same as the
total mass of the products
 That the total mass of the reactants will always be more than the
total mass of the products
 That the total mass of the reactants will always be less than the
total mass of the products
9) What mass of cobalt must have reacted if 3.20 g of oxygen forms 7.92
g of cobalt oxide?
 3.20 g
 4.72 g
 7.92 g
10) Osmium tetroxide (OsO4) is an important chemical in the synthesis
of molecules which can be used in a variety of medicines. What is the
percentage of osmium tetroxide that is made up of oxygen?
Ar of oxygen, O = 16, Ar of osmium, Os = 190.
 7.8 %
 25.2 %
 74.8 %