State law of reciprocal proportions.

Answer

**Step-by-Step Solution:**

1. **Definition of the Law of Reciprocal Proportions:**
The Law of Reciprocal Proportions states that when two different elements combine separately with a fixed mass of a third element, the ratio of the masses in which they combine will either be the same or a simple multiple of the ratio of the masses in which they combine with each other.

2. **Understanding the Elements:**
Let's consider two elements: Sulfur (S) and Oxygen (O), which will combine with a fixed mass of a third element, Copper (Cu).

3. **Assigning Atomic Masses:**
- The atomic mass of Copper (Cu) is 63.5 g.
- The atomic mass of Sulfur (S) is 32 g.
- The atomic mass of Oxygen (O) is 16 g.

4. **Combining Elements with Copper:**
- When Sulfur combines with Copper, we have the compound CuS.
- When Oxygen combines with Copper, we have the compound CuO.

5. **Mass Ratios:**
- For CuS, the mass ratio is:
- Mass of Cu = 63.5 g
- Mass of S = 32 g
- For CuO, the mass ratio is:
- Mass of Cu = 63.5 g
- Mass of O = 16 g

6. **Finding the Ratio of Sulfur and Oxygen:**
- Now, we need to find the mass ratio of Sulfur to Oxygen when they combine:
- Mass of S = 32 g
- Mass of O = 16 g
- The ratio of Sulfur to Oxygen is 32:16, which simplifies to 2:1.

7. **Comparing the Ratios:**
- The ratio of the masses in which Sulfur and Oxygen combine with Copper (Cu) is:
- CuS (32 g of S) and CuO (16 g of O).
- The ratio of Sulfur to Oxygen (2:1) is the same as the ratio of the masses in which they combine with Copper.

8. **Conclusion:**
- Therefore, according to the Law of Reciprocal Proportions, the ratios are either the same or simple multiples, which confirms the law.