Question

State the law of conservation of mass. This law applies to physical changes as well as chemical changes. With this law in mind, explain what happens to the mass of 18 mL of liquid water when it vaporizes to form about 22 L of water vapor.

Answer 1

The Law of Conservation of Mass states that, in ordinary chemical reactions, mass is neither destroyed nor created.

That is, the mass of the reactants must equal the mass of the products.

2H₂O(ℓ) ⟶ 2H₂O(g)

V/L: 22

m/g: 18

The mass of liquid water is 18 g, so the mass of the water vapour must equal 18 g.

Even though the water vapour is a gas, it still has a mass of 18 g.