Last Updated Sep 26, 2025

Amounts of Substances in Equations Simplified Revision Notes for GCSE AQA Chemistry

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Learn about Calculating Mass of Substances for your GCSE Chemistry Exam. This Revision Note includes a summary of Calculating Mass of Substances for easy recall in your Chemistry exam

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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 $(HCl)$:

$$Mg+2HCl→MgCl_2+H_2$$

• 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 grams of magnesium (Mg), and the Mr of magnesium is 24:

$$Moles of Mg= \frac{6g}{24}=0.25 moles$$

• 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.

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