State the law of definite proportion.

Explanation

**Step-by-Step Solution:**

1. **Definition of the Law of Definite Proportion**: The law of definite proportion, also known as the law of constant composition, states that in any given chemical compound, the elements are always combined in fixed proportions by mass.

2. **Explanation of the Concept**: This means that regardless of the source or how the compound is formed, the ratio of the masses of the elements in that compound will always remain the same.

3. **Example for Clarity**: For instance, in the compound glucose (C6H12O6), the elements carbon (C), hydrogen (H), and oxygen (O) are present in a specific mass ratio. No matter where glucose is obtained from, it will always contain carbon, hydrogen, and oxygen in the ratio defined by its molecular formula.

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