Question

Two basic laws of chemistry are the law of conservation of mass and the law of constant composition. Which of these laws (if any) do the following statements illustrate? (a) Lavoisier found that when mercury(II) oxide, \(\mathrm{HgO}\), decomposes, the total mass of mercury \((\mathrm{Hg})\) and oxygen formed equals the mass of mercury(II) oxide decomposed. (b) Analysis of the calcium carbonate found in the marble mined in Carrara, Italy, and in the stalactites of the Carlsbad Caverns in New Mexico gives the same value for the percentage of calcium in calcium carbonate. (c) Hydrogen occurs as a mixture of two isotopes, one of which is twice as heavy as the other.

Short answer

Answer: Statement (a) illustrates the law of conservation of mass, and statement (b) illustrates the law of constant composition.

Step-by-step solution

Statement (a)

Lavoisier observed the decomposition of mercury(II) oxide and found that the total mass of mercury and oxygen formed during the decomposition is equal to the mass of the initial mercury(II) oxide. This illustrates the law of conservation of mass, which states that mass is neither created nor destroyed in a chemical reaction. In this case, the mass of the reactants (mercury(II) oxide) is equal to the mass of the products (mercury and oxygen).

Statement (b)

The statement mentions that analysis of calcium carbonate from two different sources (Italian marble and stalactites from New Mexico) yields the same percentage of calcium in calcium carbonate. This illustrates the law of constant composition, which states that a compound always contains the same proportion of elements by mass, regardless of the source or the amount. In this case, the proportion of calcium in the calcium carbonate remains constant.

Statement (c)

Hydrogen exists as a mixture of two isotopes, one of which is twice as heavy as the other. This statement does not illustrate either the law of conservation of mass or the law of constant composition, as isotopes are forms of the same element with different atomic masses and these laws are focused on chemical reactions or compound compositions.