RPAL Conservation of Mass 2 — Questions and Answers
Question 1: In the reaction N₂ + 3H₂ → 2NH₃, if 28 g of N₂ reacts with 6 g of H₂, what mass of NH₃ is produced?
- 17 g
- 28 g
- 34 g (Correct answer)
- 40 g
Correct answer: 34 g
By conservation of mass, 28 g N₂ + 6 g H₂ = 34 g NH₃, since all reactants are consumed and mass is conserved.
Question 2: Which quantity is always equal before and after a chemical reaction in a closed system?
- Number of molecules
- Number of moles
- Total mass (Correct answer)
- Volume of substances
Correct answer: Total mass
Total mass is always conserved in a closed system because atoms are merely rearranged, not created or destroyed.
Question 3: A reaction produces 18 g of water and 44 g of CO₂ from methane and oxygen. If 16 g of methane was used, how much oxygen was consumed?
- 26 g
- 32 g
- 46 g (Correct answer)
- 62 g
Correct answer: 46 g
Total product mass = 18 + 44 = 62 g; mass of O₂ = 62 g − 16 g (CH₄) = 46 g.
Question 4: Conservation of mass requires that a chemical equation must be:
- Unbalanced
- Balanced (Correct answer)
- Written with whole number subscripts only
- Written with the same number of compounds on each side
Correct answer: Balanced
A balanced equation ensures that the number of each type of atom is equal on both sides, reflecting conservation of mass.
Question 5: If 50 g of reactants are placed in a sealed, rigid container and a reaction occurs, what is the mass of the products?
- Less than 50 g
- Exactly 50 g (Correct answer)
- More than 50 g
- It depends on the type of reaction
Correct answer: Exactly 50 g
In a sealed system, conservation of mass guarantees the total mass of products equals the total mass of reactants, which is 50 g.
Question 6: When iron rusts, it gains mass. This is consistent with conservation of mass because:
- Mass is created during oxidation
- The iron absorbs oxygen from the air, adding to the total mass (Correct answer)
- Some energy converts to mass
- The density increases, so mass increases
Correct answer: The iron absorbs oxygen from the air, adding to the total mass
Rusting is Fe + O₂ → Fe₂O₃; oxygen from air combines with iron, so the iron's mass increases while the total system mass (iron + oxygen) remains constant.
Question 7: In a decomposition reaction, CaCO₃ → CaO + CO₂, 100 g of CaCO₃ decomposes to give 56 g of CaO. What mass of CO₂ is released?
- 44 g (Correct answer)
- 56 g
- 100 g
- 144 g
Correct answer: 44 g
By conservation of mass: mass of CO₂ = 100 g − 56 g = 44 g.
RPAL Conservation of Mass 2
In the reaction N₂ + 3H₂ → 2NH₃, if 28 g of N₂ reacts with 6 g of H₂, what mass of NH₃ is produced?