An element X froms the following oxides X2O, XO and XO2. This phenomenon illustrates the law of________?

Step 1: Analyze the question Element X combines with oxygen to form three different compounds: X₂O, XO and XO₂. We need to name the law that this illustrates.

Step 2: Recall the concept

• Conservation of mass: matter is neither created nor destroyed in a chemical reaction.
• Definite proportion (constant composition): a pure compound always contains the same elements in the same proportion by mass, no matter its source.
• Mass action: the rate of a reaction is proportional to the concentrations of the reactants.
• Multiple proportion (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.

Step 3: Apply it Fix the amount of X at 2 atoms (or 2 units of mass) and compare the oxygen:

| Oxide | X | O | |---|---|---| | X₂O | 2 | 1 | | XO (X₂O₂) | 2 | 2 | | XO₂ (X₂O₄) | 2 | 4 |

The ratio of oxygen combining with a fixed amount of X is 1 : 2 : 4. This is a simple whole-number ratio, which is exactly the law of multiple proportions.

Step 4: Answer The same two elements form several different compounds, and the masses of oxygen that combine with a fixed mass of X are in simple whole-number ratios. Multiple proportion**.

Exam tip: If a question shows one pair of elements forming several compounds (like CO and CO₂, or H₂O and H₂O₂), think multiple proportions. If it shows one compound with the same composition from different sources, think definite proportion.