Four different experiments were conducted in the following ways:
I. 3 g of carbon was burnt in 8 g of oxygen to give 11 g of `CO_2`
II. 1.2 g of carbon was burnt in air to give 4.2 g of `CO_2`.
III. 4.5 g of carbon was burnt in enough air to give 11 g of `CO_2`
IV. 4 g of carbon was burnt in oxygen to form 30.3 g of `CO_2`.
Law of constant proportions is illustrated in experiment(s)

Options

A. I and III only
B. II and III only
C. IV only
D. I only.

Answer

To determine which experiments illustrate the law of constant proportions, we need to analyze the mass ratios of carbon and oxygen in each experiment. The law states that the elements in a compound are always present in a constant ratio by mass, regardless of the amount of the compound formed.

### Step-by-Step Solution:

1. **Understanding the Law of Constant Proportions**:
- The law states that in any given compound, the elements are always combined in the same proportion by mass. For example, in carbon dioxide (CO₂), the ratio of the mass of carbon to the mass of oxygen is constant.

2. **Analyzing Experiment I**:
- **Given**: 3 g of carbon is burnt in 8 g of oxygen to produce CO₂.
- **Total mass of CO₂ produced**: 3 g (C) + 8 g (O) = 11 g (CO₂).
- **Mass ratio of C to O**: 3 g C : 8 g O.
- **Molar mass of CO₂**: C (12 g) + 2 × O (16 g) = 12 g + 32 g = 44 g.
- **Ratio of C to O in CO₂**: 12 : 32 simplifies to 3 : 8.
- **Conclusion**: This experiment illustrates the law of constant proportions.

3. **Analyzing Experiment II**:
- **Given**: 1.2 g of carbon is burnt in air to produce 4.2 g of CO₂.
- **Mass of oxygen used**: 4.2 g (CO₂) - 1.2 g (C) = 3 g (O).
- **Mass ratio of C to O**: 1.2 g C : 3 g O.
- **Conclusion**: The ratio is 1.2 : 3, which simplifies to 2 : 5. This does not match the constant ratio of 3 : 8. Thus, this experiment does not illustrate the law.

4. **Analyzing Experiment III**:
- **Given**: 4.5 g of carbon is burnt in enough air to produce 11 g of CO₂.
- **Mass of oxygen used**: 11 g (CO₂) - 4.5 g (C) = 6.5 g (O).
- **Mass ratio of C to O**: 4.5 g C : 6.5 g O.
- **Conclusion**: The ratio is 4.5 : 6.5, which does not simplify to 3 : 8. Thus, this experiment does not illustrate the law.

5. **Analyzing Experiment IV**:
- **Given**: 4 g of carbon is burnt in oxygen to form 30.3 g of CO₂.
- **Mass of oxygen used**: 30.3 g (CO₂) - 4 g (C) = 26.3 g (O).
- **Mass ratio of C to O**: 4 g C : 26.3 g O.
- **Conclusion**: The ratio is 4 : 26.3, which does not simplify to 3 : 8. Thus, this experiment does not illustrate the law.

### Final Conclusion:
Only Experiment I illustrates the law of constant proportions.

### Answer:
**The law of constant proportions is illustrated in Experiment I only.**