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Every object around us has mass, which is a measure of the amount of matter it contains. When we observe everyday events, such as burning wood or mixing substances, we notice that the total mass of the materials involved does not simply disappear or appear out of nowhere. This observation leads us to a fundamental principle in chemistry known as the Law of Conservation of Mass.
This law forms the foundation of all chemical reactions and calculations. It tells us that during any chemical change, the total mass of the substances involved remains constant. Understanding this law helps chemists predict the amounts of products formed and reactants consumed in reactions, making it essential for studying chemistry at all levels.
Historically, this concept was first clearly stated and experimentally proven by Antoine Lavoisier in the 18th century, earning him the title "Father of Modern Chemistry". His work transformed chemistry from a qualitative to a quantitative science.
The Law of Conservation of Mass states:
Mass is neither created nor destroyed in a chemical reaction. The total mass of the reactants equals the total mass of the products.
What does this mean in simple terms? Imagine you have a sealed container with some substances inside that react chemically. Before the reaction, you measure the total mass of the substances (called reactants). After the reaction, the substances change into new ones (called products), but if you measure the total mass again, it will be exactly the same as before.
This law applies only to closed systems, where no matter can enter or leave the container. If the system is open, gases or solids may escape, making it seem like mass is lost, but in reality, the mass is conserved overall.
This principle allows chemists to:
Antoine Lavoisier conducted a famous experiment to prove the conservation of mass during combustion. He carefully weighed a sealed container holding a known mass of mercury oxide (HgO). When heated, mercury oxide decomposed into mercury and oxygen gas inside the container.
By weighing the container before and after heating, Lavoisier found that the total mass remained unchanged, even though the substances inside had changed form. This showed that mass was conserved during the chemical reaction.
graph TD A[Weigh sealed container with HgO] --> B[Heat container to decompose HgO] B --> C[HgO breaks into Hg + O2 inside container] C --> D[Weigh sealed container again] D --> E[Mass before = Mass after] E --> F[Conclusion: Mass is conserved]
Step 1: Write the chemical reaction:
Magnesium reacts with oxygen to form magnesium oxide:
\( 2 \text{Mg} + \text{O}_2 \rightarrow 2 \text{MgO} \)
Step 2: According to the law of conservation of mass, mass of reactants = mass of products.
Given mass of magnesium = 2.4 g
Mass of oxygen consumed is not given, but total mass of magnesium oxide will be sum of magnesium and oxygen masses.
Step 3: Assume oxygen mass is \( m \) g, then mass of magnesium oxide = 2.4 + \( m \) g.
Since the question asks for mass of magnesium oxide formed, and no oxygen mass is given, we assume oxygen is sufficient and magnesium fully reacts.
Answer: Mass of magnesium oxide = mass of magnesium + mass of oxygen consumed.
In practical problems, oxygen mass is often calculated from stoichiometry; here, the mass of magnesium oxide formed will be greater than 2.4 g by the amount of oxygen combined.
Step 1: Count atoms on both sides:
Step 2: Balance oxygen atoms by placing coefficient 2 before water:
\( \text{H}_2 + \text{O}_2 \rightarrow 2 \text{H}_2\text{O} \)
Now, right side has O = 2, H = 4.
Step 3: Balance hydrogen atoms by placing coefficient 2 before hydrogen:
\( 2 \text{H}_2 + \text{O}_2 \rightarrow 2 \text{H}_2\text{O} \)
Atoms balanced: H = 4, O = 2 on both sides.
Step 4: Calculate masses:
Answer: Mass of reactants equals mass of products (36 g), confirming the law of conservation of mass.
Step 1: Write the balanced chemical equation:
\( 2 \text{H}_2 + \text{O}_2 \rightarrow 2 \text{H}_2\text{O} \)
Step 2: Total mass of reactants = 10 g + 80 g = 90 g.
Step 3: According to the law of conservation of mass, mass of products = 90 g.
Step 4: All reactants convert to water, so mass of water formed = 90 g.
Answer: 90 g of water is formed in the sealed container.
Step 1: Calculate molar masses:
Step 2: Calculate moles of reactants:
Step 3: According to the balanced equation, 1 mole N₂ reacts with 3 moles H₂ to produce 2 moles NH₃.
Given reactants are in exact stoichiometric ratio, so all reactants convert to products.
Step 4: Calculate mass of ammonia produced:
Moles of NH₃ formed = 2 moles
Mass = moles x molar mass = \( 2 \times 17 = 34 \) g
Answer: 34 g of ammonia is produced.
Step 1: Understand the system:
Wood burning is an open system reaction where gases like carbon dioxide and water vapor escape into the air.
Step 2: The mass of ash is less because some products (gases) leave the system.
Step 3: The law of conservation of mass applies to a closed system where no matter escapes.
Step 4: If the gases released were captured and weighed along with the ash, total mass would equal the original wood mass.
Answer: No violation occurs; apparent mass loss is due to the open system allowing gases to escape. The law holds true in closed systems.
When to use: To avoid errors related to mass loss due to gases escaping or external interactions.
When to use: When solving stoichiometry problems or chemical reaction calculations.
When to use: To prevent confusion between mass and energy conservation laws.
When to use: During calculations to avoid unit conversion errors.
When to use: When conceptualizing or explaining the law to beginners.
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