A compound XO exists in nature. A scientist wanted to prepare the same compound in his lab based on the law of constant proportion but the formula of the compound that he prepared was found to be XO2. Which postulates of the law of constant proportion did he violate?

The postulate that all pure samples of a particular chemical compound contain the same elements  combined in the same proportion by mass.

Explanation:

The law of constant composition (or definite proportion) states that all pure samples of a particular chemical compound contain the same elements  combined in the same proportion by mass.

This law implies that irrespective of the source of the sample of the compound or the method by which the sample was prepared, as long as the sample is pure, the elements present in the compound are always combined in a specific ratio by mass. For example, water has the molecular formula H₂O. Irrespective of the water source, for example, tap-water, lake, stream, river or rainfall, each pure sample of water will have the constant composition of 2 atoms of hydrogen to one atom of oxygen.

The given compound exists as XO in nature. Therefore, irrespective of the source or method of preparation, every sample of this compound should have the formula XO. Since the compound prepared by the scientist was XO₂, therefore, the postulate that all pure samples of a particular chemical compound contain the same elements  combined in the same proportion by mass was violated by the scientist. The scientist must have prepared a different compound of the elemental compositions.

im not exactly sure but i think its b!

they are all solids.