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Year 10
AQA
Higher

Mass in a chemical reaction: using moles

I can interpret a balanced symbol equation to predict the mass of either a reactant or a product.

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New
New
Year 10
AQA
Higher

Mass in a chemical reaction: using moles

I can interpret a balanced symbol equation to predict the mass of either a reactant or a product.

Download all resources
Share activities with pupils

Lesson details

Key learning points

  1. In a chemical reaction, the atoms in reactants are rearranged and are the same atoms that are in the products.
  2. Chemical equations are most correctly interpreted in terms of molar ratios (i.e. 1 mole of A reacts with 2 moles of B).
  3. The molar ratios of the substances in a chemical reaction is known as stoichiometry, which means ‘measuring elements’.
  4. The stoichiometry of a reaction is shown by coefficients in a balanced equation. These molar relationships don't change.
  5. The moles of substance in a reaction can be determined using: mass(g) = Mr × moles (& the stoichiometry of reaction).

Common misconception

Pupils struggle to recall the order of steps required to mathematically process the available information in order to answer the question.

Colour-coding the steps can help pupils remember how many steps are involved. Much practice and perseverance is needed to create and maintain the memory pathway of the mathematical processing.

Keywords

  • Stoichiometry - Stoichiometry refers to the molar ratio of the reactants and products in a chemical reaction.

  • Relative formula mass - The relative formula mass (RFM or Mᵣ) of a substance is the sum of the relative atomic masses of all the atoms in its formula.

  • Mole - 1 mole of a substance is 6.02 × 10²³ particles of it. The mass of a mole of a substance is its relative mass in grams.

  • Coefficient - A coefficient is the number placed in front of a chemical formula to balance an equation; it multiplies all the atoms in the formula and shows the ratio of substances in a reaction.

  • Balanced symbol equation - A balanced symbol equation represents a chemical reaction using symbols and coefficients to ensure that the number of atoms of each element is equal on both sides of the equation.

Use of modelling via visualisers, colour-coding and faded 'I do, you do' examples is particularly useful to help pupils develop a memory strategy for the mathematical processing required to answer these types of questions. Much practice and perseverance is required to build confidence with this.
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Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.
Given the relative atomic masses (Ar): C (12), O (16) Calculate the relative molecular mass (Mr) of carbon dioxide (CO₂).
Correct answer: 44
28
192
0.75
Q2.
What does the conservation of mass in a chemical reaction imply?
Mass of products is always more than the mass of reactants.
Mass of reactants is converted into energy.
Correct answer: Mass of reactants equals mass of products.
Atoms are destroyed to conserve mass.
Q3.
If 20 g of sodium reacts fully with 15 g of chlorine, how much sodium chloride is produced?
Correct Answer: 35 g, 35, 35 grams
Q4.
For the equation __ N₂ + __ H₂ → __ NH₃, select the correct coefficients to balance it.
Correct answer: 1, 3, 2
1, 2, 3
1, 3, 3
2, 3, 2
Q5.
What does the equation 2H₂ + O₂ → 2H₂O tell us?
Two molecules of hydrogen react with two molecules of oxygen.
Correct answer: Two molecules of hydrogen react with one molecule of oxygen.
One molecule of hydrogen reacts with one molecule of oxygen.
Two molecules of water are decomposed into hydrogen and oxygen.
Q6.
Given the relative atomic mass values (Ar): H (1), S (32), O (16) Calculate the relative formula mass (Mr) of sulfuric acid.
Correct Answer: 98

Exit quiz

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6 Questions

Q1.
What does stoichiometry mean in chemistry?
The study of chemical reactions.
Correct answer: Measuring elements and their ratios in reactions.
The temperature changes in chemical reactions.
The speed of chemical reactions.
Q2.
In a balanced chemical equation, what does the mole ratio of reactants and products tell us?
The speed of the reaction.
The physical states of the reactants and products.
How many particles are involved in the reaction.
Correct answer: The proportions of substances that react or are produced.
Q3.
The is the sum of the relative atomic masses of all the atoms in a formula, representing the mass of a molecule relative to the mass of a carbon-12 atom.
Correct Answer: relative formula mass, RFM, Mr
Q4.
Which of the following statements best describes what the equation 2H₂ + O₂ → 2H₂O tells us?
Two molecules of hydrogen react with one molecule of oxygen.
Correct answer: Two moles of hydrogen are needed to react with one mole of oxygen.
Four hydrogen atoms react with two oxygen atoms.
2 hydrogens reacts with oxygen to make 2 lots of water.
Q5.
Given the reaction: CH₄ + 2O₂ → CO₂ + 2H₂O, how many grams of CO₂ are produced when 16 g of methane (CH₄) reacts completely? Relative formula mass values: (CH₄ = 16) & (CO₂ = 44)
22 g
Correct answer: 44 g
66 g
88 g
Q6.
Using the reaction: N₂ + 3H₂ → 2NH₃, calculate the mass of ammonia (NH₃) produced when 14 g of nitrogen react with hydrogen? (2 s.f.) Relative formula mass values: (N₂ = 28) & (NH₃ = 17)
Correct Answer: 17 g, 17 grams, 17