Introduction:
The law of constant composition, also known as the law of definite proportions, states that a chemical compound always contains the same elements in the same proportions by mass. This law is a fundamental principle in chemistry and provides a basis for understanding the composition and behavior of compounds. In this case, we will analyze the results of two experiments involving the compound cupric oxide (CuO) to determine if they illustrate the law of constant composition.

Experiment 1: Reduction of Cupric Oxide:
- Mass of cupric oxide before reduction: 2.75g
- Mass of copper remaining after reduction: 2.196g

Experiment 2: Conversion of Copper Nitrate to Cupric Oxide:
- Mass of copper used: 2.358g
- Mass of cupric oxide formed: 2.952g

Analysis:
To determine if the results of these experiments illustrate the law of constant composition, we need to compare the ratios of copper to oxygen in cupric oxide in both experiments.

Calculations:

Experiment 1:
- Mass of copper in cupric oxide before reduction = 2.75g - 2.196g = 0.554g
- Mass of oxygen in cupric oxide = 2.75g - 0.554g = 2.196g
- Ratio of copper to oxygen = (0.554g / 2.196g) ≈ 0.252

Experiment 2:
- Mass of oxygen in cupric oxide = 2.952g - 2.358g = 0.594g
- Ratio of copper to oxygen = (2.358g / 0.594g) ≈ 3.973

Comparison:
If the results of the experiments illustrate the law of constant composition, the ratio of copper to oxygen in cupric oxide should be the same. However, in this case, the ratio in Experiment 1 is approximately 0.252, while the ratio in Experiment 2 is approximately 3.973. These ratios are significantly different, indicating that the law of constant composition is not observed in this example.

Explanation:
The discrepancy in the ratios of copper to oxygen in cupric oxide between the two experiments suggests that the composition of cupric oxide is not consistent. This could be due to experimental errors, variations in the reaction conditions, or impurities in the starting materials. The law of constant composition holds true for pure substances under specific conditions, but in practical scenarios, deviations can occur.

Conclusion:
Based on the analysis of the experimental results, it can be concluded that the law of constant composition is not illustrated in this case. The ratios of copper to oxygen in cupric oxide obtained from the two experiments are significantly different, indicating a lack of consistency in the composition of the compound. It is important to recognize that while the law of constant composition is a fundamental principle in chemistry, there can be exceptions and variations in real-world scenarios.