Worksheet: Conservation of Mass

In this worksheet, we will practice using the law of conservation of mass to calculate the masses of reactants and products in a chemical reaction.


Yeast converts glucose to ethanol and carbon dioxide by anaerobic fermentation, as represented by the equation shown. Glucoseethanolcarbondioxideโ†’+ In a particular fermentation process, 200.0 g of glucose is fully converted.

What is the total mass of ethanol and carbon dioxide at the end of the reaction?

If the fermentation process were carried out in an open container, would the mass of the container and contents increase, decrease, or stay the same?

  • ADecrease
  • BIncrease
  • CStay the same

If the reaction produces 97.7 g of carbon dioxide, what mass of ethanol is produced?


Antoine Lavoisier was one of the first scientists to state the law of conservation of mass. When he heated a sample of tin and air in an airtight flask, tin reacted with oxygen in the air to produce tin oxide. Did the mass of the flask and its contents increase, decrease, or stay the same because of this reaction?

  • AIncreased
  • BStayed the same
  • CDecreased


Bread dough is placed in a baking tin and baked at 180โˆ˜C. What happens to the mass of the bread?

  • AIt increases.
  • BIt decreases.
  • CIt stays the same.


A piece of magnesium ribbon is burned in air, producing white powdered magnesium oxide. Is the mass of the oxide greater than, less than, or equal to that of the magnesium ribbon it came from?

  • AGreater than
  • BEqual to
  • CLess than


Why do we use an objectโ€™s mass, rather than its weight, to indicate the amount of matter it contains?

  • AWeight depends on gravity, which depends on where you are in the universe. Mass is universal.
  • BMass depends on gravity, which is fixed.
  • CMass is more accurate than weight.
  • DThe numbers we get for mass are smaller and easier to handle.
  • EActually, we should use weight but mass is easier to measure.


A wet 43.2 g sample of copper sulfate heptahydrate (CuSOยท7HO)42 is heated until only copper sulfate (CuSO)4 remains. The mass of the water lost is 34.1 g. What is the mass of the copper sulfate?


A 13.5 g sample of calcium carbonate is heated until it decomposes completely and 7.6 g of calcium oxide is produced. What is the mass of carbon dioxide produced?


Magnesium burns in air to produce magnesium oxide. If a sample of magnesium requires 15.0 g of oxygen to burn completely and forms 37.8 g of magnesium oxide, what is the mass of the sample of magnesium?


A 100 g sample of tin undergoes a phase change from solid to liquid. Is the mass of liquid tin equal to, less than, or greater than the mass of tin in the original sample?

  • AGreater than 100 g
  • BEqual to 100 g
  • CLess than 100 g


An open-top beaker contains 15.3 g of ethanol. A lump of sodium of 7.6 g is dropped into the beaker and reacts completely, producing sodium ethoxide (CHONa)25.

What is the mass of sodium ethoxide expected to be?

The actual mass of sodium ethoxide produced is found to be 22.6 g. By writing a balanced equation for the reaction, which of the following statements explains why the mass appears to have decreased?

  • AThe reaction uses oxygen from the air that is not measured in the reactants.
  • BThe reaction produces carbon dioxide gas that escapes from the beaker.
  • CThe reaction has changed the state of sodium from a solid to an aqueous product.
  • DThe reaction destroys some atoms of hydrogen.
  • EThe reaction produces hydrogen gas that escapes from the beaker.


Sodium bicarbonate (X) can be made according to the reaction shown. NaCl+CO+NH+HOX+NHCl2324 Use the conservation of mass principle to answer the following.

Determine the relative formula mass of product X. Give your answer as a whole number.

What is the molecular formula for X?

  • ANaCO23
  • BNaCO3
  • CNaO32
  • DNaHCO3
  • ENaHCO2


Nickel reacts with a certain number of moles of carbon monoxide to produce an organonickel compound. The equation for this chemical reaction can be written as shown. Ni+CONi(CO)๐‘›๐‘›5.9 g of nickel is found to produce 17.1 g of Ni(CO)๐‘›. Determine the value of ๐‘› to the nearest whole number.


The combustion of toluene proceeds according to the reaction shown. Tolueneoxygencarbondioxidewater+โ†’+92 g of toluene was found to produce 308 g of carbon dioxide and 72 g of water. How many moles of oxygen were required for the complete combustion of toluene?


Which of these statements about the conservation of mass is not correct?

  • AThe mass of the products will be equal to the mass of the reactants.
  • BMass is not conserved during a physical exchange (e.g., melting).
  • CDuring a chemical reaction, no mass is created or destroyed.
  • DDuring a chemical reaction, no atoms are created or destroyed.
  • EThere will be the same number and type of atoms before and after a chemical reaction.


A chemical reaction is represented by the general equation shown. xAB+4C The chemical reaction is found to produce 223 g of B and 68 g of C. If the molar mass of A is 97 g/mol, determine ๐‘ฅ, the number of moles of A.

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