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Chemistry · Grades 9–12

Conservation of mass in reactions

Study guide for Grades 9 to 12: 2 worked examples and 2 practice questions. Atoms are only rearranged in a reaction, so mass is conserved. If mass seems to change, a gas has escaped or been taken in from the air.

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Rule and key idea

Atoms are only rearranged in a reaction, so mass is conserved. If mass seems to change, a gas has escaped or been taken in from the air.

in a closed system: total mass of reactants = total mass of products

Worked examples

In a sealed container, 28.6 g of substance A reacts completely with 7.7 g of substance B. The only product is substance C. What mass of C forms?

28.6 + 7.7 = 36.3

36.3 g

In a sealed container, 6.9 g of A and 27 g of B react until B is used up. 2.1 g of A is left over, and the only product is C. What mass of C forms?

6.9 + 27 − 2.1 = 31.8

31.8 g

Try two questions

  1. A log burns in an open fireplace, and the ash left over has much less mass than the log. What is the best explanation?

    1. Gases such as carbon dioxide and water vapour escaped into the air.
    2. The fire destroyed some of the matter.
    3. Ash is lighter because it is grey.

  2. In a sealed container, 9.4 g of A and 5.9 g of B react until B is used up. 0.9 g of A is left over, and the only product is C. What mass of C forms?

Answers and reasoning

Answers and marking notes
  1. Gases such as carbon dioxide and water vapour escaped into the air.

    Mass is conserved; most of the products left as gases, which were not weighed.

  2. 14.4 g

    9.4 + 5.9 − 0.9 = 14.4

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