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Reacting MassesOxford AQA IGCSE Chemistry: Revision notes

Section 1

What is conservation of mass?

Chemical reactions can be represented by word equations or balanced symbol equations, using state symbols (g), (l), (s) and (aq) where appropriate.

During a chemical reaction, no atoms are lost or made — atoms are simply rearranged. This means the total mass of the products equals the total mass of the reactants (conservation of mass).

Because mass is conserved, chemists can use a balanced symbol equation to calculate the mass of one substance from the mass of another.

Key termsconservation of mass

Section 2

How do you calculate reacting masses using a balanced equation?

To calculate the mass of a reactant or product:

  1. Write the balanced symbol equation
  2. Calculate the relative formula mass (Mr) of the substance you know and the substance you want to find
  3. Convert the known mass into moles: n = mass ÷ Mr
  4. Use the mole ratio from the balanced equation to find the moles of the unknown substance
  5. Convert the moles of the unknown substance back into mass: mass = n × Mr
Key termsmole ratio
Example

Calculate the mass of magnesium oxide produced when 6 g of magnesium burns completely. 2Mg + O2 → 2MgO (Ar: Mg = 24, O = 16, Mr(MgO) = 40). Moles Mg = 6 ÷ 24 = 0.25. Mole ratio Mg:MgO is 1:1, so moles MgO = 0.25. Mass MgO = 0.25 × 40 = 10 g.

Section 3

How do you use mole ratios from unbalanced-looking equations?

The large numbers in front of formulae in a balanced equation give the mole ratio between substances.

For example, in 2Mg + O2 → 2MgO, 2 moles of Mg react with 1 mole of O2 to produce 2 moles of MgO. If the ratio is not 1:1, you must multiply or divide the moles of the known substance accordingly to find the moles of the unknown substance.

Common mistake

A common error is using the mass ratio instead of the mole ratio from the equation — always convert to moles first, apply the ratio, then convert back to mass.

Section 4

Why might the actual yield differ from the calculated (theoretical) yield?

In practice, it is not always possible to obtain the full calculated mass of product because:

  • The reaction may not go to completion
  • Some product may be lost during separation (e.g. filtering or transferring liquids)
  • Reactants may take part in unexpected side reactions

Must Know

  • No atoms are lost or made in a chemical reaction, so total mass of reactants = total mass of products
  • To find reacting masses: convert known mass to moles, apply the mole ratio from the balanced equation, then convert back to mass
  • n = mass ÷ Mr, and mass = n × Mr
  • The numbers in front of formulae in a balanced equation give the mole ratio
  • Actual yield can be lower than calculated yield due to incomplete reactions, losses during separation, or side reactions

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