Calcium burns in oxygen to form calcium oxide. A student investigates this reaction and states that the law of conservation of mass means the total mass of products must equal the total mass of reactants. Explain what is meant by the law of conservation of mass and describe how the student could use the balanced equation 2Ca + O₂ → 2CaO to show that the masses of reactants and products are consistent with this law.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Calcium (relative atomic mass = 40) reacts with oxygen gas (relative atomic mass of O = 16) according to the equation: 2Ca + O₂ → 2CaO. A student burns a sample of calcium completely in excess oxygen and collects all of the calcium oxide formed.
Model answer (5 marks)
The law of conservation of mass states that no atoms are lost or made during a chemical reaction.
The mass of the products must equal the mass of the reactants.
Using the balanced equation 2Ca + O₂ → 2CaO:
1. 2Ca has a relative formula mass of 2 × 40 = 80.
2. O₂ has a relative formula mass of 2 × 16 = 32.
Total reactant mass = 80 + 32 = 112.
3. 2CaO has a relative formula mass of 2 × (40 + 16) = 112.
Total product mass = 112.
Because the total mass of the products (112) equals the total mass of the reactants (112), the reaction obeys the law of conservation of mass.
The mass of the products must equal the mass of the reactants.
Using the balanced equation 2Ca + O₂ → 2CaO:
1. 2Ca has a relative formula mass of 2 × 40 = 80.
2. O₂ has a relative formula mass of 2 × 16 = 32.
Total reactant mass = 80 + 32 = 112.
3. 2CaO has a relative formula mass of 2 × (40 + 16) = 112.
Total product mass = 112.
Because the total mass of the products (112) equals the total mass of the reactants (112), the reaction obeys the law of conservation of mass.
Examiner tips
- State the law in your own words first. Show the calculation for each side of the equation. Use the exact relative atomic masses given. Conclude that the masses are equal to confirm the law.
Common mistakes
- Using the wrong atomic mass for oxygen (e.g. 8 instead of 16). Failing to double the mass of CaO when calculating the product mass. Not explicitly stating that the total masses are equal to demonstrate conservation.
Mark scheme (5 marks)
- The law of conservation of mass states that no atoms are lost or made during a chemical reaction
- The mass of the products equals the mass of the reactants
- The relative formula mass of 2Ca is correctly stated as 2 × 40 = 80
- The relative formula mass of O₂ is correctly stated as 32, giving total reactant mass of 80 + 32 = 112
- The relative formula mass of 2CaO is 2 × (40 + 16) = 112, which equals the total reactant mass, confirming conservation of mass
Key terms in this question
law of conservation of mass · reactants · products
Related
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