The law of constant proportions, which states that a chemical compound always contains the same elements in the same fixed ratio by mass, was first clearly enunciated by which scientist?

Introduction / Context:
One of the foundational ideas in chemistry is that a pure compound always contains the same elements combined in the same fixed proportion by mass, regardless of its source or method of preparation. This principle is called the law of constant proportions or the law of definite proportions. It played a crucial role in the development of atomic theory. The question asks you to identify the scientist who first clearly formulated and enunciated this law.

Given Data / Assumptions:

• The law states that a given chemical compound always has the same composition by mass.
• Several notable chemists are listed as options: Dalton, Berthelot, Avogadro, and Proust.
• We assume familiarity with basic historical developments in early chemistry.
• We must match the correct name to this specific law.

Concept / Approach:
Joseph Proust, a French chemist, performed careful analyses of compounds such as copper carbonate and found that they always contained the same mass ratio of elements, no matter how they were prepared. From these experiments he concluded that chemical compounds are defined by definite proportions. This became known as Proust law of definite proportions or law of constant composition. John Dalton later used this and other laws to develop atomic theory but is more closely associated with the law of multiple proportions. Avogadro is known for Avogadro hypothesis about equal volumes of gases. Berthelot made contributions in thermochemistry. Therefore, the correct answer is Joseph Proust.

Step-by-Step Solution:
Step 1: State the law of constant proportions: a given compound always contains the same elements in the same ratio by mass. Step 2: Recall that Joseph Proust experimentally analysed various samples of the same compound prepared in different ways and found identical composition. Step 3: Recognise that he generalised these observations into the law of definite proportions. Step 4: Compare this with John Dalton contributions, which focused on atomic theory and the law of multiple proportions, not constant proportions. Step 5: Identify Proust among the options as the chemist associated with the law of constant proportions.

Verification / Alternative check:
Historical accounts of the development of chemical laws attribute the law of definite proportions to Proust around the late eighteenth and early nineteenth centuries. Dalton built on Proust and Lavoisier work to propose that atoms combine in simple whole number ratios, explaining these mass relationships. Avogadro later introduced his famous hypothesis relating gas volumes and numbers of molecules. Berthelot contributed to thermochemistry and synthesis of organic compounds. None of these other scientists are typically credited with establishing the law of constant proportions itself, confirming that Proust is the correct choice.

Why Other Options Are Wrong:
John Dalton did propose several important ideas about atoms and is linked with the law of multiple proportions, which states that when two elements form more than one compound, the ratios of the masses of one element that combine with a fixed mass of the other are in simple whole numbers. However, he did not first enunciate the law of constant proportions. Avogadro is remembered for Avogadro law about gas volumes and mole concept, not for constant proportions. Berthelot is associated with thermochemistry and synthetic organic chemistry but not with this law. Thus, these options do not correctly answer the question.

Common Pitfalls:
Students often confuse the various classical chemists because their work is interrelated and appears in the same chapter of textbooks. It is easy to attach the wrong law to a familiar name such as Dalton or Avogadro. To avoid confusion, it is helpful to link Proust with fixed composition, Dalton with multiple proportions and atomic theory, Avogadro with gas volumes and the mole, and Lavoisier with conservation of mass. These associations make it easier to recall which scientist is linked to which major law.

Final Answer:
The law of constant proportions was first clearly enunciated by Joseph Proust.