Amount of Substance Calculations (Edexcel A Level Chemistry): Revision Note

Amount of Substance Calculations

• As previously discussed, the amount of substance can be defined as the number of particles in a substance, n, measured in moles (often abbreviated to mol)

• In reality, amount of substance is used as a blanket term to cover most chemical calculations, especially those that involve moles

• The two main calculations for amount of substance are:

Moles, n = $\frac{mass,m}{molarmass,M}$

Moles, n = concentration x volume

• Other common calculations for amount of substance include:

Moles, n = $\frac{numberofparticles}{Avogadro\text{'}sconstant}$

Moles, n =$\frac{PV}{RT}$from the ideal gas equation (PV = nRT)

Reacting Mass Calculations

• Chemical equations can be used to calculate the moles or masses of reactants and products

• To do this, information given in the question is used to find the amount in moles of the substances being considered

• Then, the ratio between the substances is identified using the balanced chemical equation

• Once the moles have been determined they can then be converted into grams using the relative atomic or relative formula masses

Balancing Equations using Reacting Masses

• If the masses of reactants and products of a reaction are known then we can use them to write a balanced equation for that reaction

• This is done by converting the masses to moles and simplifying to find the molar ratios