A metal M forms two types of chlorine, MCl$_2$ and MCl$_3$ which of the following laws best explains the relationship between the chlorides? Law of

Analyze the question: A single metal M combines with chlorine to form two different compounds, MCl₂ and MCl₃. We need the law that describes this relationship.

Concept: the laws of chemical combination

• Conservation of mass: Matter is neither created nor destroyed in a chemical reaction.
• Definite proportions (constant composition): A given compound always contains the same elements in the same proportion by mass.
• Multiple proportions (Dalton): 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 simple whole-number ratio.
• Reciprocal proportions: If two elements each combine with a third element, the ratio in which they do so is related to the ratio in which they combine with each other.

Solution: Take a fixed amount of M, say 1 mole.

• In MCl₂, it combines with 2 moles of Cl, which is 2 × 35.5 = 71 g.
• In MCl₃, it combines with 3 moles of Cl, which is 3 × 35.5 = 106.5 g.

Ratio of chlorine masses = 71 : 106.5 = 2 : 3, a simple whole-number ratio.

This is exactly what the law of multiple proportions states: the same two elements form two different compounds, and the masses of one element (Cl) combining with a fixed mass of the other (M) are in a simple ratio.

The other options do not fit:

• Definite proportion applies to one compound, not to two different compounds.
• Conservation of mass concerns mass in a reaction, not the relationship between two compounds.
• Reciprocal proportion needs three different elements, and here there are only two.

Correct answer: C. multiple proportion

Exam tip: Look at the number of compounds. One compound with fixed composition points to definite proportion. Two or more compounds from the same two elements (such as CO and CO₂, or H₂O and H₂O₂) point to multiple proportions. Three elements point to reciprocal proportions.