**The Law of Constant Proportions**

The law of constant proportions, also known as the law of definite proportions, states that a chemical compound always contains the same elements in the same proportion by mass. This means that regardless of the source or method of preparation, a compound will always have a fixed composition.

**Calculation of Oxygen in CaO**

According to the given information, 2.8 grams of CaO contain 0.8 grams of oxygen. To determine the proportion of oxygen in CaO, we can use the following calculation:

Oxygen in CaO = (Oxygen in CaO / Total mass of CaO) * 100

= (0.8 grams / 2.8 grams) * 100

= 28.57%

Therefore, it can be concluded that the proportion of oxygen in CaO is approximately 28.57%.

**Calculation of Oxygen in Calcium**

Next, we can determine the proportion of oxygen in calcium by using the given information that 1 gram of oxygen yields 3.5 grams of CaO.

Oxygen in CaO = (Oxygen in CaO / Total mass of CaO) * 100

= (1 gram / 3.5 grams) * 100

= 28.57%

Again, we find that the proportion of oxygen in CaO is approximately 28.57%, which is the same as the proportion in the previous calculation.

**Comparison of Results**

By comparing the proportion of oxygen in both CaO and calcium, we can see that they are equal. This demonstrates the law of constant proportions, as the same elements are present in the same proportion by mass in both compounds.

Since CaO is formed by the reaction between calcium and oxygen, the consistent proportion of oxygen in both compounds supports the law of constant proportions. The law implies that the ratio of calcium to oxygen will always be the same, regardless of the source or method of preparation.

This law is a fundamental principle in chemistry and provides a basis for understanding the composition and behavior of chemical compounds. It allows scientists to predict the composition of compounds and analyze their properties.

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