Explanation:
The given statement states that irrespective of the source, a pure sample of water always yields 88.89% mass of oxygen and 11.11% mass of hydrogen. This ratio of oxygen to hydrogen is consistent, regardless of the source from which the water sample is obtained. This is explained by the law of constant composition.

Law of Constant Composition:
The law of constant composition, also known as the law of definite proportions, states that a given chemical compound always contains the same elements in the same proportion by mass. This means that the ratio of the masses of the elements in a compound is always constant.

Application of the Law of Constant Composition to Water:
Water is a compound composed of two elements, oxygen (O) and hydrogen (H). According to the law of constant composition, the ratio of the masses of oxygen to hydrogen in water is always constant.

In the given statement, it is stated that irrespective of the source, a pure sample of water always yields 88.89% mass of oxygen and 11.11% mass of hydrogen. This means that for every 88.89 grams of oxygen in the water sample, there will be 11.11 grams of hydrogen.

This consistent ratio of oxygen to hydrogen in water is a direct application of the law of constant composition. It means that no matter where the water sample is obtained from, whether it is from a river, lake, or ocean, the ratio of oxygen to hydrogen will always be the same.

Importance of the Law of Constant Composition:
The law of constant composition is a fundamental principle in chemistry. It allows chemists to accurately determine the composition of compounds and predict their properties based on the elements present.

In the case of water, the law of constant composition allows scientists to determine the mass ratio of oxygen to hydrogen in a water sample. This is important for various applications, such as water purification, chemical analysis, and understanding the behavior of water in different chemical reactions.

Conclusion:
The given statement that a pure sample of water always yields 88.89% mass of oxygen and 11.11% mass of hydrogen is explained by the law of constant composition. According to this law, a compound always contains the same elements in the same proportion by mass. In the case of water, the ratio of oxygen to hydrogen is always constant, regardless of the source of the water sample.