Conservation of Mass (College Board AP® Chemistry)
Study Guide
Written by: Martín
Reviewed by: Stewart Hird
Conservation of Mass
Mass cannot be created or destroyed, but it can be rearranged
The Law of conservation of mass states the mass must state the same over time
During chemical processes, the atoms in chemical compounds are rearranged to form new ones
The principle of mass conservation can be used to calculate the amount of products or the amount of reactants for a given chemical equation
Worked Example
The following chemical reaction is carried in a chemical reactor:
2H2 + O2 → 2H2O
If 4 g of hydrogen gas were used during the reaction, and 36 g of water were collected at the end of the reaction. How much oxygen gas reacted?
Answer:
Step 1: Count the amount of grams of reactants and products. If there is an unknown mass, place it as an unknown (x)
Mass of the reactants (g) | Mass of the products (g) |
4 + x | 36 |
Step 2: Set up a mathematical equation using the law of conservation of mass
mass of reactants = mass of products
4 + x = 36
Step 3: Solve the equation to find the unknown mass of oxygen
4 + x = 36
x = 36-4 =32
32 g of O2 reacted
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