Conservation of Mass — Practice · Grade 8 Science

Chapter: Chemistry

What this practice covers

By the end of Conservation of Mass you should be able to:

10 questions, numeric, multiple choice and written answer, each with a worked explanation.

Vocabulary used: law of conservation of mass, closed system, model, evidence, system boundary, assumption.

Problem 1

12 g of carbon reacts with 32 g of oxygen to form carbon dioxide. What is the mass of carbon dioxide produced?
Answer and explanation

44 g

By conservation of mass: 12 g + 32 g = 44 g of carbon dioxide.

Problem 2

In a reaction, 25 g of reactant A combines with reactant B to form 60 g of product. What was the mass of reactant B?
Answer and explanation

35 g

60 g (products) − 25 g (reactant A) = 35 g of reactant B.

Problem 3

The law of conservation of mass says that in a chemical reaction…
  1. mass can be created but not destroyed
  2. the total mass of the reactants equals the total mass of the products
  3. products always weigh less than the reactants
  4. atoms are destroyed and new ones form
Answer and explanation

the total mass of the reactants equals the total mass of the products

Conservation of mass means reactant mass and product mass are always equal in a closed system.

Problem 4

Explain, in terms of atoms, why mass is conserved in a chemical reaction.
Answer and explanation

Atoms are only rearranged into new combinations (different molecules); no atoms are created or destroyed, so the total mass before and after the reaction stays the same.

Problem 5

5.0 g of magnesium burns in oxygen to form 8.3 g of magnesium oxide. How many grams of oxygen combined with the magnesium?
Answer and explanation

3.3 g

Oxygen used = 8.3 g (product) − 5.0 g (magnesium) = 3.3 g.

Problem 6

An open reaction starts with 80 g and leaves 77 g of material. If the only loss is gas, what mass escaped? Explain the reasoning in your own words.
Answer and explanation

3 g of gas; remaining material plus gas still totals 80 g.

Use initial mass=remaining mass+escaped mass.

Subtract 80−77=3 g.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

An open setup starts at 92 g and ends at 87 g. Assuming gas is the only loss, calculate escaped mass. Show the reasoning, not just the conclusion.
Hints

Count all products, including invisible gas.

Answer and explanation

92−87=5 g; remaining material plus escaped gas totals 92 g.

Problem 8

A sealed reaction reads 0.2 g less afterward. Name two checks before concluding mass conservation failed.
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Check balance uncertainty and calibration, then leaks or spills and whether the same container parts were included. A measurement mismatch requires investigation.

Problem 9

New substances form when the same atoms regroupNew substances form when the same atoms regroupReactants: 2H₂ + O₂Products: 2H₂OBefore and after: 4 H atoms and 2 O atoms. New combinations form.Blue = H; red = O. Counting model only; no hydrogen ignition activity.
This supplied water-forming reaction contains four hydrogen atoms and two oxygen atoms on each side. Their combinations change from H₂ and O₂ to H₂O. Colored circles distinguish atom types rather than literal colors, and the bonds are counting aids. It is a different example from a bath-bomb reaction; the figure does not claim that a bath bomb contains hydrogen gas or oxygen gas.
For this unit’s illustrated case, what evidence in the diagram supports a chemical change without supporting creation of new atoms?
Answer and explanation

The product molecules have a different composition/combination from the reactant molecules, while each element’s atom count stays four H and two O.

A reactant is a starting substance and a product is a resulting substance. Different combinations can give different characteristic properties. The drawing is an explanation for an already specified reaction, not proof that every visible bubble is a new substance.

The product molecules have a different composition/combination from the reactant molecules, while each element’s atom count stays four H and two O.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 10

A specified product has mass 14 g after reacting with 9 g of the initial solid in an open apparatus. If incoming gas accounts for the change, how much gas mass must be included in a complete boundary?
Answer and explanation

5 g of incoming gas, so 9+5 = 14 g. The increase concerns matter crossing the measured boundary rather than matter being created.

Subtract the initial solid mass from product mass.

Identify the initially omitted matter source.

5 g of incoming gas, so 9+5 = 14 g. The increase concerns matter crossing the measured boundary rather than matter being created.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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