Law of Conservation of Mass

Introduction to the Law

The Law of Conservation of Mass is a fundamental concept in chemistry, first articulated by Antoine Lavoisier in 1789. It states that in a chemical reaction, matter is neither created nor destroyed; instead, it is conserved. This means the total mass of the reactants equals the total mass of the products.

• Definition: In any chemical reaction, the mass of the substances produced (products) is equal to the mass of the substances that react (reactants).

• Implication: Atoms are rearranged during chemical reactions, but their total number and mass remain unchanged.

Chemical Equation Example

The reaction between hydrogen and oxygen to form water is a classic example:

• Reactants: Hydrogen gas and oxygen gas

• Product: Liquid water

Application of the Law

• According to Lavoisier, all masses are accounted for in a chemical reaction.

• Mass of reactants = Total mass of products

Worked Example

Example: How many grams of water vapor will form if 2.50 grams of hydrogen gas reacts with 12.0 grams of oxygen gas?

• Given: 2.50 g H2 + 12.0 g O2

• Find: Mass of H2O produced

(all mass is converted to water vapor)

Practice Problems

• Problem 1: Predict the minimum amount of nitrogen that will react with 50.0 grams of hydrogen to produce 53.0 grams of ammonia.

• Solution:

  • Let x = mass of nitrogen

  • Mass of reactants = mass of products

• Problem 2: Find the amount of oxygen gas that will remain after the reaction of 112.4 grams of calcium with 240 grams of oxygen.

• Reaction:

  • Calcium + Oxygen → Calcium Oxide

  • 112.4 g + 240 g → 156 g

  • Oxygen remaining:

Summary Table: Conservation of Mass in Reactions

Additional info: The Law of Conservation of Mass is foundational for balancing chemical equations and for quantitative chemical calculations in stoichiometry.