To write the formula of a compound, you have to know the valencies of the elements in that compound. You use the periodic table to find valencies eg;

Valency 1 2 3 4 5 6 7 0


Formula of Aluminum Oxide

symbol  Al  O
valency 3   2
swap the valencies;
The formula is Al2O3.   

Valency Table of common compounds

Name Formula Valency
Hydroxides OH 1
Nitrate NO3 1
sulphate SO42- 2
Carbonate CO32- 2

Examples of worked formulae of compounds

Formula of Aluminum hydroxide

Balance their charges
Aluminum hydroxide Al3+(=+3)  OH- *3(=-3)
The formula is AlOH3.   

Formula of calcium nitrate

Balance their charges
Calcium nitrate Ca2+(=+2)    NO3- *2(=2)
The formula is Ca(NO3)2.   

Formula of Calcium phosphate

Balance their charges Calcium phosphate Ca2+ *3(=+6) PO43- *2(=-6) The formula is Ca3(PO4)2.

Chemical Equations

A chemical equation shows the reactants and products in a reaction and may include state symbols, which show the physical states of each substance. Solid, liquid and gaseous states are represented by the state symbols (s), (l) and (g) respectively. Substances that are dissolved in water to form an aqueous solution are represented with the state symbol (aq).

Balanced equations

A balanced is when there are equal number of atoms and charges on both sides of the equation. State symbols are solid(s), liquid(l),aqueous solution(aq) and gas(g). A word equation has reactants and product(s).

Writing equations

  • write the word equation in words first eg;
  • magnesium + oxygen → magnesium oxide
  • Simply write the symbol in an equation;
  • Mg + O2 → MgO
  • Balance the equation, numbers need to be written in front of a reactant and or product to balance the equation;
  • 2Mg + O2 → 2MgO
  • Write chemical symbols;
  • 2Mg(s) + O2(g) → 2MgO(s)

Examples of balanced equations

  • 2Na(s) + Cl2(g) → 2NaCl(s)
  • 2Fe(s) + 3Br2(g) → 2FeBr3(s)
  • Na(s) + H2O(l) → NaOH(aq) + H2(s)

Ionic Equations

Ionic equations are chemical equations that show the reaction involving ions. It is important to note the physical states of all substances in the reaction


Relative atomic mass (Ar)

Mass of one atom of an element relative to one twelfth of the mass of one atom of Carbon-12 eg Ar(Al)= 27 and Ar(Ca) =40. They are found using the periodic table

Relative formula mass (Mr)

sum of relative atomic masses of all the atoms in one molecule of the compound. Examples are given below;

Calculate the relative formula mass of Ca(OH)2 and CH4

Ca(OH)2 = 40 + [2( 16 + 1)] = 74
CH4 = 12 + (1*4) =16


A mole of a substance is the amount that contains the same number of units as the number of carbon atoms in 12 grams of carbon-12.

Calculating number of moles

  • Number of moles = mass/molar mass
  • Number of moles (gase) =volume/ 24dm3
  • Number of moles of solute = concentration (mol/dm3) * volume (dm3



  • Concentration = Moles/ volume( dm3); in mol/dm3. 1dm3= 1000cm3


Empirical Formulae

This is the simplest ratio of the atoms in a compound.

  • How to calculate the empirical formulae
    • Make the percent ratio into the simplest whole number ratio (NOTE: if given percentages, use them as grams)
    • Divide the coefficients of each element symbol by the lowest coefficient

Molecular formula

Show the actual number of atoms of each element in one molecule of a substance. It is the multiple of the empirical formula

Calculate the empirical and molecular formulae of a hydrogen compound which contains 92.3% carbon and 7.7% hydrogen with molecular mass of 78

Empirical formula
Percentage composition          Carbon(92.3)        Hydrogen(7.7)
Divide by Ar of element         92.7/12 = 7.69      7.7/1 = 7.7 
Divide by smallest no. of moles  7.69/7.69 = 1       7.7/7.69 = 1
Ratio                               1                   1
Therefore the empirical formula is CH

Molecular formula
Relative molecular mass = 12(Ar of C)n + 1(Ar of H)n
13n = 78
n = 6

Therefore the molecular formula is C6H6



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