Understanding the Law of Conservation of Mass

The question asks us to identify the correct statement about the law of conservation of mass from the given options. Let's examine each option to determine which one accurately describes this fundamental principle in chemistry.

The law of conservation of mass is a foundational concept stating that mass is neither created nor destroyed in a closed system during any chemical or physical change. This means the total mass of the reactants before a chemical reaction must equal the total mass of the products after the reaction.

Analyzing the Options

Let's look at each statement:

• Option 1: "A given compound always contains exactly same proportion of elements". This statement describes the Law of Definite Proportions (also known as the Law of Constant Composition). This law states that a pure chemical compound always contains its component elements in a fixed proportion by mass. For example, water (\(\text{H}_2\text{O}\)) always contains hydrogen and oxygen in a mass ratio of $1:8$. This is not the law of conservation of mass.
• Option 2: "When gases combine in a reaction, they do so in a simple ratio by volume, provided all gases are at room temperature." This statement relates to Gay-Lussac's Law of Gaseous Volumes. It states that when gases react with each other, they do so in volumes that bear a simple whole-number ratio to one another, and to the volume of the product, if gaseous, provided that the temperature and pressure are the same for all gases. This is not the law of conservation of mass.
• Option 3: "Matter can neither be created nor destroyed". This statement is the precise definition of the Law of Conservation of Mass. It directly expresses the principle that the total amount of matter remains constant during a chemical transformation or physical process in a closed system.
• Option 4: "Equal volumes of all gases at same temperature and pressure contain equal numbers of molecules." This statement is Avogadro's Law. It states that equal volumes of all gases, at the same temperature and pressure, have the same number of molecules. This law helps relate the volume of a gas to the number of moles or molecules, but it is not the law of conservation of mass.

Identifying the Correct Statement

Based on the analysis, the statement that correctly describes the law of conservation of mass is "Matter can neither be created nor destroyed".

Therefore, option 3 is the correct answer.

Revision Table: Laws of Chemical Combination

Additional Information: Chemical Reactions and Mass Conservation

The law of conservation of mass is crucial for understanding stoichiometry in chemical reactions. When we write a balanced chemical equation, we are implicitly applying this law. The number of atoms of each element on the reactant side must equal the number of atoms of that same element on the product side. Since atoms have mass, conserving the number of atoms for each element ensures that the total mass is conserved.

For example, consider the reaction between hydrogen gas (\(\text{H}_2\)) and oxygen gas (\(\text{O}_2\)) to form water (\(\text{H}_2\text{O}\)):

\(\text{2H}_2\text{(g) + O}_2\text{(g) \(\rightarrow\) 2H}_2\text{O(l)}\)

On the reactant side, we have \(2 \times 2 = 4\) hydrogen atoms and $2$ oxygen atoms. On the product side, we have \(2 \times 2 = 4\) hydrogen atoms and \(2 \times 1 = 2\) oxygen atoms. The number of atoms of each element is conserved, and consequently, the total mass is conserved.

This law was formulated by Antoine Lavoisier in the late 18th century and is a cornerstone of classical chemistry.