The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. This implies, in a closed system, the mass of the elements involved initially in a chemical reaction is equal to the mass of the product obtained by the reaction. This concept of mass conservation is widely used in chemistry and other fields like mechanics, dynamics, etc.
A system that is isolated from its surroundings is one that does not interact with it.
- As a result, no matter what changes or chemical reactions occur in that isolated system, the mass will remain constant; even if the final state may differ from the initial state, there can be no more or less mass than before the change or reaction.
- For example, kinetic energy is converted to potential energy when a toy vehicle is rolled down a ramp and collides with a wall.
Formula
Mass of Reactants = Mass of Products
However, in fluid mechanics and continuum mechanics, the law of conservation of mass can be represented as follows using the differential form of the continuity equation:
\frac{\delta \rho}{\delta t}+▽(\rho v)=0 where,
ρ is the density,
t is the time,
v is the velocity, and
∇ is the divergence.
This law helps us explain what happens to mass during physical and chemical changes as follows:
Physical Change
When the matter undergoes a physical change, the law of conservation of mass holds true.
- Place a small amount of ice, which is just frozen water, in a flask.
- The flask is now being gently heated to melt the ice into the water after being appropriately weighted and overfilled.
Ice → (Heat) → Water
Chemical Change
When the matter undergoes a chemical change, the law of conservation of mass holds true.
- Take one mole of sodium hydroxide (NaOH) and one mole of hydrochloric acid (HCl) and allow them to react in a test tube.
- After the reaction, you will notice that one mole of sodium chloride (NaCl) and one mole of water (H₂O) are formed, which establishes the law of conservation of mass.
Examples
1. Formation of Water
Reaction:
2H₂ + O₂ → 2H₂O
- Hydrogen molecular mass = 2 g/mol (H ₂)
- Oxygen molecular mass = 32 g/mol (O ₂)
- Water molecular mass = 18 g/mol (H₂O)
- 4 g of Hydrogen + 32 g of Oxygen → 36 g of Water
- Total mass before reaction = 36 g
- Total mass after reaction = 36 g
- Mass is conserved.
2. Carbon Dioxide
Reaction:
C + O₂ → CO₂
- 12 g of Carbon + 32 g of Oxygen → 44 g of CO₂
- Total mass before reaction = 44 g
- Total mass after reaction = 44 g
- Mass is conserved.
Limitation
- A nuclear reaction produces an imbalance between the mass of the reactants and the mass of the products because some of the mass is transformed into energy.
- If a chemical reaction is carried out in an open container, gases may escape, giving the impression that mass is not conserved.
- The law does not consider the interconversion of mass and energy, which is explained in modern physics.
Solved Examples
Example 1: If heating 12.0 grams of calcium carbonate (CaCO₃) produces 6.4 g of carbon dioxide (CO₂) and 5.6 g of calcium oxide (CaO), show that these observations are in agreement with the law of conservation of mass.
Solution:
We know that,
Mass of the reactants = Mass of the products
12.0g of CaCO3 = 6.4g of CO2 + 5.6g of CaO
12.0g of reactant = 12.0g of products
Because the mass of the reactant is equal to the mass of the products, the observations are in agreement with the law of conservation of mass.
Example 2: Sodium carbonate reacts with ethanoic acid to form sodium ethanoate, carbon dioxide, and water. In an experiment, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid to form 8.2 g of sodium ethanoate, 2.2 g of carbon dioxide, and 0.9 g of water. Show that this data verifies the law of conservation of mass.
Solution:
We know,
Sodium Carbonate + Ethanoic Acid = Sodium Ethanoate + Carbon Dioxide + Water.
Substituting the masses of the compounds we have,
5.3g + 6g = 8.2g + 2.2g + 0.9g = 11.3g
Hence, the conservation of mass is verified.
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