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(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

(a) When mercury(II) oxide decomposes, the total mass of mercury and oxygen formed is equal to the mass of mercury(II) oxide decomposed. (b) The percentage of calcium in calcium carbonate is the same in marble mined from Italy and in stalactites in the Carlsbad Caverns of New Mexico. (c) Hydrogen occurs as a mixture of two isotopes with different masses. Answer: (a) Law of Conservation of Mass (b) Law of Constant Composition (c) Neither

Step-by-step solution

Statement (a) analysis

This statement states that when mercury(II) oxide, \(\mathrm{HgO}\), decomposes, the total mass of mercury (Hg) and oxygen formed is equal to the mass of mercury(II) oxide decomposed. This implies that the mass of reactants is equal to the mass of products, which aligns with the Law of Conservation of Mass.

Statement (b) analysis

This statement states that the percentage of calcium in calcium carbonate is the same regardless of whether it is found in marble mined in Carrara, Italy, or in the stalactites of the Carlsbad Caverns in New Mexico. This means that the proportions of elements in this compound are constant and don't vary depending on their source, which conforms to the Law of Constant Composition.

Statement (c) analysis

This statement states that hydrogen occurs as a mixture of two isotopes with different masses. This information is related to the isotopes of an element and their abundance but does not demonstrate a relation between either the conservation of mass in a chemical reaction or the constant proportion of elements found in compounds. Hence, it does not illustrate either the Law of Conservation of Mass or the Law of Constant Composition.

Key concepts

Law of Conservation of Mass

The Law of Conservation of Mass is a fundamental principle in chemistry. This law states that mass is neither created nor destroyed in a closed system during a chemical reaction. What this means is that the total mass of the reactants equals the total mass of the products. For instance, if you start a reaction with 10 grams of reactants, you will end up with 10 grams of products. This law is all about keeping track of every atom in the reactants and ensuring they all show up in some form in the products.

Let's consider the breakdown of mercury(II) oxide as an example: When \[\mathrm{HgO} \rightarrow \mathrm{Hg} + \mathrm{O}_2\]happens, the mass of mercury and oxygen formed is identical to the mass of mercury(II) oxide that decompose. This illustrates the Law of Conservation of Mass, ensuring that no mass is lost or gained during the reaction process.

Law of Constant Composition

The Law of Constant Composition, also known as the Law of Definite Proportions, is another key chemical principle. It asserts that any given chemical compound always contains its component elements in a fixed ratio by mass. This means that no matter where or how the compound is sourced or how much of it there is, the composition in terms of element mass ratios remains constant.

For example, calcium carbonate (CaCO₃) found in marble from Italy and stalactites from New Mexico have the same percentage composition of calcium, carbon, and oxygen. This consistency demonstrates the Law of Constant Composition. So, even though the source of the calcium carbonate might change, its elemental makeup remains the same, ensuring predictability in chemical compositions.

Element Isotopes

Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers.

Hydrogen, for example, has isotopes like protium and deuterium. Protium has just one proton, while deuterium, an isotope of hydrogen, has one proton and one neutron, making it twice as heavy as protium. It's important to note that isotopes are not just limited to hydrogen; many elements have isotopes and they often share similar chemical properties. Isotopes play a vital role in various fields, including medicine, archaeology, and chemistry, as they help scientists understand elemental variations and date archaeological finds.

Mass of Reactants and Products

Understanding the mass of reactants and products in a chemical reaction is critical for mastering chemical equations and reactions. When discussing masses in chemical reactions, the conservation principles apply to ensure that all atoms in the reactants are accounted for in the products.

When you perform a chemical reaction, the mass of the substances you start with (reactants) should exactly equal the mass of the substances you end up with (products), assuming the system is closed. This concept closely ties into the law of conservation of mass, backing it up with practical significance in laboratory and industrial settings.

By measuring the mass of reactants and products, one can verify chemical equations and ensure that the reaction has been balanced correctly. Balancing equations is essential for predicting reaction results and calculating the needed quantities of reactants for desired products.