Write one limitation of law of definite proportions.

Explanation

**Step-by-Step Text Solution:**

1. **Understanding the Law of Definite Proportions**: The law states that a chemical compound always contains its component elements in fixed ratio by mass, regardless of the source or how it was prepared.

2. **Identifying the Limitation**: One significant limitation of the law of definite proportions is its applicability to compounds that consist of more than one isotope of an element.

3. **Explaining the Limitation**: In cases where an element has multiple isotopes (atoms of the same element with different numbers of neutrons), the mass ratio of the elements in the compound can vary depending on the isotopic composition. This means that the fixed proportion by mass may not hold true.

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