Amount of Substance — GCSE Chemistry Revision
Revise Amount of Substance for GCSE Chemistry. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel, OCR, WJEC, Eduqas, CCEA, Cambridge International (CIE), SQA, IB, AP.
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Go to Concentration of SolutionsWhat is Amount of Substance?
The amount of substance is a measure of the number of particles in that substance, with the mole as the standard unit. This concept is used to calculate the quantities of reactants and products in chemical reactions, based on the stoichiometry of the balanced chemical equation.
Board notes: Reacting mass calculations are a significant part of the higher-tier chemistry papers for AQA, Edexcel, and OCR. You must be able to follow the logical steps and use balanced equations to determine the amount of product or reactant.
Step-by-step explanationWorked example
What mass of magnesium oxide is produced from 6g of magnesium? (2Mg + O₂ → 2MgO). 1. Moles of Mg = Mass / Ar = 6g / 24 = 0.25 mol. 2. From the equation, the ratio of Mg to MgO is 2:2 or 1:1. So, 0.25 mol of MgO is produced. 3. Mass of MgO = Moles x Mr = 0.25 x (24+16) = 0.25 x 40 = 10g.
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Common mistakes
- 1Not using the ratio from the balanced chemical equation when calculating reacting masses.
- 2Forgetting to convert mass into moles before using the stoichiometric ratio.
- 3Mixing up the steps in a reacting mass calculation. The sequence is: Mass → Moles → Ratio → Moles → Mass.
Amount of Substance exam questions
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Amount of Substance
Core concept
The amount of substance is a measure of the number of particles in that substance, with the mole as the standard unit. This concept is used to calculate the quantities of reactants and products in che…
Frequently asked questions
What is a limiting reactant?
The limiting reactant is the reactant that gets completely used up in a chemical reaction. It determines the maximum amount of product that can be formed.
How do you find the limiting reactant?
To find the limiting reactant, you calculate the number of moles of each reactant and compare it to the ratio in the balanced equation. The reactant that produces the least amount of product is the limiting one.