The law which states that a pure chemical compound, no matter how it is made, will be made up of the same elements contained in the same proportion by mass is

Analysis

The question asks for the specific scientific law that defines the composition of a pure chemical compound in terms of the fixed mass ratios of its constituent elements, regardless of the method of preparation.

Concept Explanation

In chemistry, the laws of chemical combination describe how elements combine to form compounds. Here is a breakdown of the options provided:

• Law of Conservation of Mass: States that matter is neither created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.
• Law of Definite Proportion (also known as the Law of Constant Composition): States that a given chemical compound always contains its component elements in a fixed ratio by mass, regardless of its source or method of preparation. For example, water ($H_2O$) will always have hydrogen and oxygen in a mass ratio of 1:8, whether it comes from a river, a laboratory synthesis, or a volcanic vent.
• Law of Multiple Proportion: States that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers.
• Law of Reciprocal Proportion: States that if two different elements combine separately with a fixed mass of a third element, the ratio of the masses in which they do so are either the same or a simple multiple of the ratio in which they combine with each other.

Step-by-Step Solution

1. Identify the key phrase: The question specifies "a pure chemical compound... made up of the same elements contained in the same proportion by mass."
2. Match with definitions: This definition is the textbook description of the Law of Definite Proportion, proposed by Joseph Proust.
3. Evaluate options: • Option A relates to mass balance in reactions. • Option B matches our definition perfectly. • Option C relates to multiple compounds formed by the same elements. • Option D relates to the ratios of elements combining with a third element.

Conclusion

The law described is the Law of Definite Proportion.

Exam Tip

To remember this easily, think of the word "Definite" as "Fixed." If the proportion is fixed or definite, it doesn't matter where the substance came from; the recipe for that compound remains exactly the same!