# Mole Concept and Stoichiometry – Formulae for O Level Chemistry

In this post, you will find a summary of all the mole concept and stoichiometry formulae you’ll need for your O Level Chemistry exams.

### 1. Converting number (of particles e.g. ions, atoms, molecules ,etc) to number of moles

number of particles = Avogadro’s Constant x number of moles

where Avogadro’s Constant = 6.02 x 1023

### 2. Converting mass to moles

moles = mass / molar mass

### 3. Converting volume of gas to moles

moles = volume of gas / molar volume

where molar volume = 24 dm3 at room temperature and pressure (r.t.p)

### 4. Converting concentration of solution to moles

moles = concentration (in mol/dm3) x volume (dm3)

### 5. Converting between different units of concentration g/dm3 to mol/dm3

concentration (g/dm3) = concentration (mol/dm3) x molar mass

### 6. Finding mass from concentration (in g/dm3)

mass (g) = concentration (g/dm3) x volume (dm3)

### 7. Stoichiometry Rules

The number (or coefficient) in front of each compound or element in a balanced equation represent the mole ratio required for the reaction. If there is no number in front of the element or compound, then it is 1.

Let’s use the equation below as an example:

2HCl + Ba(OH)2 –> BaCl2 + 2H2O

Based on the equation above, it tells us that the stoichiometry ratio of HCl to Ba(OH)2 in the reaction is 2:1. This means that 2 moles of HCl will react with 1 mole of Ba(OH)2 . If 0.3 moles of HCl is used, then 0.15 mole of Ba(OH)2 is needed for the reaction.

Key concept: Chemical reaction gives us the mole ratio of the reactants and products.

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