The law of conservation of mass states that in a closed system, the total mass of the system remains constant over time, regardless of the processes occurring within the system. In other words, mass cannot be created or destroyed; it can only change form.
Example:
Consider the reaction of hydrogen gas with oxygen gas to form water:
\[ 2H_2(g) + O_2(g) \rightarrow 2H_2O(l) \]
Before the reaction:
- We have 2 moles of hydrogen gas (H₂) and 1 mole of oxygen gas (O₂).
- The molar mass of H₂ is about 2 g/mol, so 2 moles of H₂ have a mass of 4 grams.
- The molar mass of O₂ is about 32 g/mol, so 1 mole of O₂ has a mass of 32 grams.
- Total mass before reaction = 4 g + 32 g = 36 g.
After the reaction:
- The product formed is water (H₂O).
- The molar mass of H₂O is about 18 g/mol, and since the reaction produces 2 moles of H₂O, the total mass of water will be:
- Mass of 2 moles of H₂O = 2 x 18 g/mol = 36 g.
Total mass after the reaction = 36 g.
In this example, the total mass of reactants (hydrogen and oxygen) equals the total mass of the product (water), thus illustrating the law of conservation of mass.