Two samples X and Y of a pure substance obtained by two different methods contain elements P and Q. Substance X has a mass of 2.25 g and contains 0.90 g of Q. Substance Y is made up of 60% P and 40% Q by weight. This is an illustration of

Options

A. Law of multiple proportions
B. Law of constant proportions
C. Law of conservation of mass
D. None of these

Explanation

To solve the question, we need to analyze the information provided about the two samples, X and Y, and determine what principle or law it illustrates.

### Step-by-Step Solution:

1. **Identify the Mass of Substance X**:
- The mass of substance X is given as 2.25 g.

2. **Determine the Mass of Element Q in Substance X**:
- The mass of element Q in substance X is given as 0.90 g.

3. **Calculate the Mass of Element P in Substance X**:
- To find the mass of element P in substance X, we subtract the mass of Q from the total mass of X:
\[ \text{Mass of P in X} = \text{Total mass of X} - \text{Mass of Q} = 2.25 \, \text{g} - 0.90 \, \text{g} = 1.35 \, \text{g} \]

4. **Determine the Composition of Substance Y**:
- Substance Y is composed of 60% P and 40% Q by weight. This means that if we assume a total mass of 100 g for simplicity:
- Mass of P in Y = 60 g
- Mass of Q in Y = 40 g

5. **Calculate the Ratio of P to Q in Both Substances**:
- For substance X:
\[ \text{Ratio of P to Q in X} = \frac{\text{Mass of P}}{\text{Mass of Q}} = \frac{1.35 \, \text{g}}{0.90 \, \text{g}} = 1.5 \]
- For substance Y:
\[ \text{Ratio of P to Q in Y} = \frac{\text{Mass of P}}{\text{Mass of Q}} = \frac{60 \, \text{g}}{40 \, \text{g}} = 1.5 \]

6. **Compare the Ratios**:
- Both substances X and Y have the same ratio of P to Q, which is 1.5. This indicates that the proportion of elements in the compounds is constant, regardless of the method of preparation.

7. **Identify the Law Illustrated**:
- The fact that the ratio of elements P and Q remains constant in both samples illustrates the **Law of Constant Proportions** (also known as the Law of Definite Proportions). This law states that a chemical compound always contains the same proportion of elements by mass.

### Conclusion:
The correct answer to the question is that this is an illustration of the **Law of Constant Proportions**.