“A given compound always contains exactly the same proportion of elements by weight” is stated under which law? 

CONCEPT:

Law of Definite Proportions

• The Law of Definite Proportions, also known as the Law of Constant Composition, states that a given chemical compound always contains its component elements in fixed ratio by mass, regardless of the source or method of preparation.
• This law ensures that the composition of a compound is consistent and does not vary.

EXPLANATION:

• The statement “A given compound always contains exactly the same proportion of elements by weight” directly reflects the essence of the Law of Definite Proportions.
• For example, water (H₂O) always consists of 2 parts hydrogen and 16 parts oxygen by weight, which corresponds to a fixed ratio of 1:8.
• This law is different from other laws such as:
  • Law of Conservation of Mass: It states that mass is neither created nor destroyed in a chemical reaction.
  • Law of Multiple Proportions: It states that when two elements combine to form multiple compounds, the ratios of the masses of one element that combine with a fixed mass of the other element are simple whole numbers.
  • Avogadro's Law: It relates the volume of gases to the number of molecules under constant temperature and pressure.

Therefore, the correct answer is Law of Definite Proportions.