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Year 10
AQA
Foundation

Mass in a chemical reaction

I can use conservation of mass and relative formula masses to predict the mass of unknown products or reactants.

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New
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Year 10
AQA
Foundation

Mass in a chemical reaction

I can use conservation of mass and relative formula masses to predict the mass of unknown products or reactants.

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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. The mass of products in a chemical reaction is equal to the mass of the reactants.
  3. The sum of the RFM of the reactants equals the sum of the RFM of the products, taking account of balancing coefficients.

Keywords

  • 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.

  • Conservation of mass - Conservation of mass means the total mass of the starting reactants equals the total mass of the products formed.

  • 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.

  • 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.

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.

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.
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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).

Lesson video

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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 20g of sodium reacts fully with 15g of chlorine, how much sodium chloride is produced?
Correct Answer: 35 g, 35 grams, 35
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.
Match the following terms with their correct definitions:
Correct Answer:balanced symbol equation,describes the numbers of atoms taking part in a reaction

describes the numbers of atoms taking part in a reaction

Correct Answer:conservation of mass,in a chemical reaction mass is neither created nor destroyed

in a chemical reaction mass is neither created nor destroyed

Correct Answer:relative formula mass (RFM),the sum of the relative atomic masses of all atoms in a formula

the sum of the relative atomic masses of all atoms in a formula

Correct Answer:coefficient,the number placed in front of a formula in an equation

the number placed in front of a formula in an equation

Q2.
True or false? In a chemical reaction, the total mass of all the products can be different to the total mass of all the reactants
true
Correct answer: false
Q3.
Balance the following equation: __ H₂ + __ O₂ → __ H₂O.
H₂ + O₂ → H₂O
H₂ + O → H₂O
Correct answer: 2H₂ + O₂ → 2H₂O
2H₂ + 2O₂ → 2H₂O
2H + O → H₂O
Q4.
2Mg + O₂ → 2MgO. Relative atomic mass (Ar): Mg (24), O (16). 48 g of magnesium reacts with excess oxygen. Calculate the mass of MgO produced.
40 g
Correct answer: 80 g
64 g
48 g
Q5.
2Na + Cl₂ → 2NaCl. Relative atomic mass (Ar): Na (23), Cl (35.5). 46 g of Na reacts with excess chlorine. Calculate the mass (g) of NaCl produced (3 s.f.).
Correct Answer: 117 g, 117 grams, 117
Q6.
H₂ + SO₃ → H₂SO₄. Relative atomic mass (Ar): H (1), S (32), O (16). 8.0 g of SO₃ reacts with excess hydrogen. Calculate the mass (g) of H₂SO₄ produced (2 s.f.).
Correct Answer: 9.8 g, 9.8, 9.8 grams