Each element in a given compound comprises a fixed fraction or perfecnt by

According to law of definite composition, a chemical compound is always made up of elements in same ratio by mass or a compound always have a fixed composition. For example, in water H_{2}O there is always 1 atom of oxygen with mass 16 g/mol and 2 atoms of hydrogen with mass 1 g/mol each. It has application for both molecular and ionic compounds.

Thus, each element in a given compound comprises a fixed fraction or percent by mass.

Answer: Elements combine in a fixed ratio by their mass.

Explanation:

Law of definite proportion states that the elements combining to form compounds always combine in a fixed ratio by their mass.

For Example: Taking a compound known as water

Here, 2 grams of hydrogen combines with 16 grams of oxygen.

So, in every molecule of water, hydrogen and oxygen will combine in a fixed ratio of 1 : 8 by their mass.

Hence, elements combine in a fixed ratio by their mass.