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The law of definite proportions, also known as the law of constant composition, states that a chemical compound always contains the same elements in the same proportions by mass, regardless of the amount or source of the compound. In other words, the atoms of each element combine in a specific and consistent ratio to form a compound.

The chemical reaction of hydrogen and oxygen combining to form water can be written as: \(2 H_2 (g) + O_2 (g) \rightarrow 2 H_2O(l)\) Here, two moles of gaseous hydrogen react with one mole of gaseous oxygen to produce two moles of liquid water.

To understand how the law of definite proportion applies to this reaction, let's first determine the mass proportion of hydrogen and oxygen in water. Water has a molecular formula of H2O, which means it consists of 2 hydrogen atoms (H) and 1 oxygen atom (O). The atomic mass of hydrogen is approximately 1 gram per mole, while the atomic mass of oxygen is approximately 16 grams per mole. Therefore, the mass proportion of hydrogen to oxygen in water is approximately: \(M_{H_2O} = 2 \times M_{H} + 1 \times M_{O}\)

No matter the amount of hydrogen and oxygen reacted, the resulting water will always have the same proportion of hydrogen to oxygen by mass, as dictated by the law of definite proportion. In this reaction, the definite proportion is a ratio of 2:1 in moles (or 2g of hydrogen to 16g of oxygen by mass). Even if there is an excess of one reactant, the product will always have a consistent composition, with the same proportions of hydrogen and oxygen. In conclusion, the composition of water formed from the reaction between hydrogen and oxygen does not depend on the amount of oxygen reacted, following the law of definite proportion. The atoms of hydrogen and oxygen in water always combine in a specific, consistent ratio that is not affected by the amount of reactants involved in the reaction.