Definite Proportions Law Explained

The question asks us to identify the specific law that states a compound always contains the same elements combined in the same proportion by mass. This is a fundamental concept in chemistry related to the composition of substances.

Understanding the Law of Definite Proportions

The Law of Definite Proportions, often associated with the work of Joseph Proust, precisely describes this phenomenon. It states that:

• A given chemical compound always consists of the same elements.
• These elements are always combined in the same proportion by mass.

For example, water (H₂O) is always composed of hydrogen and oxygen. Regardless of whether you obtain water from a tap, a river, or by synthesizing it, the ratio of the mass of hydrogen to the mass of oxygen is always constant. Specifically, water is approximately 11.1% hydrogen and 88.9% oxygen by mass. This fixed ratio is a defining characteristic of a specific compound.

Analyzing Other Chemistry Laws

Let's look at why the other options are not the correct fit for the question:

• Law of Conservation of Mass: This law states that mass is neither created nor destroyed during a chemical reaction. It deals with the overall mass balance in reactions, not the specific proportional composition of a single compound.
• Law of Multiple Proportions: This law applies when two elements form more than one compound. It states that in such cases, the different masses of one element that combine with a fixed mass of the other element are in the ratio of small whole numbers. For instance, comparing Carbon Monoxide (CO) and Carbon Dioxide (CO₂). This law deals with *multiple* compounds from the same elements, not the fixed composition of a *single* compound.
• Avogadro's Law: This law relates the volume of gases to the number of molecules under constant temperature and pressure. It doesn't directly address the mass proportions of elements within a compound.

Therefore, the law that specifically states a compound contains the same elements in the same proportion by mass is the Law of Definite Proportions.