Amounts of Substances in Equations (AQA GCSE Chemistry Combined Science): Revision Notes
📚 Revision Notes
3.2.2 Amounts of Substances in Equations
Moles in Chemical Equations
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When you look at a balanced chemical equation, the numbers in front of the chemical formulas tell you the ratio of moles of each substance involved in the reaction.
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Example: In the reaction between magnesium (Mg) and hydrochloric acid :
- This equation tells us that 1 mole of magnesium reacts with 2 moles of hydrochloric acid to produce 1 mole of magnesium chloride (MgCl₂) and 1 mole of hydrogen gas (H₂).
Calculating Moles from Mass
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To find out how many moles of a substance you have, divide the mass of the substance by its Mr (or Ar for elements):
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Example: If you have 6 grammes of magnesium (Mg), and the Mr of magnesium is 24:
- Once you know the moles, you can figure out how much of each substance is needed or produced in a reaction using the mole ratios from the balanced equation.