Law of Conservation of Mass Practice

What is the Law of Conservation of Mass?

In simple terms, the Law of Conservation of Mass states that mass cannot be created or destroyed in a chemical reaction.

Think of it like building with LEGO bricks. If you have a pile of red bricks and a pile of blue bricks and you use them all to build a house, the final house will weigh the exact same amount as the two piles of bricks did separately. You haven't made any new LEGOs or lost any old ones; you've just rearranged them!

In chemistry, the atoms are like the LEGO bricks. A chemical reaction just rearranges them into new molecules (the products), but the total mass you start with (the reactants) must equal the total mass you end up with (the products).

The Golden Rule: Total Mass of Reactants = Total Mass of Products

Practice Problems

Use the Law of Conservation of Mass to solve the following problems. Show your work!

Problem 1:

If 12 grams of carbon (C) react completely with 32 grams of oxygen (O₂) to form carbon dioxide (CO₂), what is the total mass of the carbon dioxide produced?

Problem 2:

In a reaction, 58 grams of butane (C₄H₁₀) are burned with oxygen to produce 176 grams of carbon dioxide and 90 grams of water. What mass of oxygen was needed for this reaction?

Problem 3:

When heated, calcium carbonate (CaCO₃) decomposes into calcium oxide (CaO) and carbon dioxide (CO₂). If 100 grams of calcium carbonate are heated and produce 56 grams of calcium oxide, what mass of carbon dioxide was produced?

Problem 4:

Sodium metal reacts with water to produce sodium hydroxide and hydrogen gas. If 46 g of sodium reacts with 36 g of water to produce 80 g of sodium hydroxide, what mass of hydrogen gas is released?

Answers 

Problem 1:

Reactants: Carbon (12 g) + Oxygen (32 g)

Products: Carbon Dioxide (?)

Mass of Reactants = Mass of Products

12 g + 32 g = ?

Answer: 44 g of carbon dioxide were produced.

Problem 2:

Reactants: Butane (58 g) + Oxygen (?)

Products: Carbon Dioxide (176 g) + Water (90 g)

Total Mass of Products = 176 g + 90 g = 266 g

Mass of Reactants = Mass of Products

58 g + ? = 266 g

? = 266 g - 58 g

Answer: 208 g of oxygen were needed.

Problem 3:

Reactants: Calcium Carbonate (100 g)

Products: Calcium Oxide (56 g) + Carbon Dioxide (?)

Mass of Reactants = Mass of Products

100 g = 56 g + ?

? = 100 g - 56 g

Answer: 44 g of carbon dioxide were produced.

Problem 4:

Reactants: Sodium (46 g) + Water (36 g)

Products: Sodium Hydroxide (80 g) + Hydrogen Gas (?)

Total Mass of Reactants = 46 g + 36 g = 82 g

Mass of Reactants = Mass of Products

82 g = 80 g + ?

? = 82 g - 80 g

Answer: 2 g of hydrogen gas were released.