The Law of Conservation of Mass

Definition and Historical Context

The Law of Conservation of Mass is a fundamental principle in chemistry, first articulated by Antoine Lavoisier in 1789. It states that in a chemical reaction, matter is neither created nor destroyed. Instead, atoms are rearranged to form new substances, but the total mass remains constant throughout the reaction.

• Reactants: The starting substances in a chemical reaction.

• Products: The substances formed as a result of the reaction.

• In a balanced chemical equation, the total mass of the reactants equals the total mass of the products.

Application in Chemical Equations

To illustrate the Law of Conservation of Mass, consider the reaction of hydrogen and oxygen to form water:

• Balanced equation:

• Here, 2 molecules of hydrogen gas react with 1 molecule of oxygen gas to produce 2 molecules of water vapor.

• The total mass of the reactants equals the total mass of the products.

Example Calculation

Example: How many grams of water vapor will form if 25.0 grams of hydrogen gas mix with 12.0 grams of oxygen gas?

• Given reaction:

• Mass of hydrogen: 25.0 g

• Mass of oxygen: 12.0 g

• Total mass of reactants:

• By the Law of Conservation of Mass, mass of water vapor produced = 37.0 g

Formula:

Stoichiometry and Predicting Masses in Reactions

Stoichiometric Calculations

Stoichiometry involves using balanced chemical equations to calculate the masses of reactants and products involved in a chemical reaction. The Law of Conservation of Mass is the foundation for these calculations.

Practice Problem 1: Nitrogen and Hydrogen to Ammonia

Problem: Predict the minimum amount of nitrogen that will react with 5.00 grams of hydrogen to produce 92.5 grams of ammonia.

• Balanced equation:

• Given: 5.00 g hydrogen, 92.5 g ammonia

• Calculated minimum amount of nitrogen required: 4.625 g

Practice Problem 2: Calcium and Oxygen Reaction

Problem: Predict the amount of oxygen gas that will remain after the reaction of 112.6 grams of calcium with 24.0 grams of oxygen.

• Balanced equation:

• Given: 112.6 g calcium, 24.0 g oxygen

• Calculated mass of calcium oxide produced: 125.6 g

• Calculated mass of oxygen remaining: 2.7 g

Summary Table: Mass Relationships in Example Reactions

Key Takeaways

• The Law of Conservation of Mass is essential for balancing chemical equations and performing stoichiometric calculations.

• Always ensure the total mass of reactants equals the total mass of products in a chemical reaction.

• Stoichiometry allows chemists to predict the amounts of substances consumed and produced in reactions.