Final answer:
The law of definite proportions dictates that a chemical compound always has the same proportion of elements by mass, which is crucial for stoichiometry, consistency in chemical formulations, and predictability in chemical reactions. The composition of water, for example, is always the same regardless of its source, due to this law.
Step-by-step explanation:
Understanding the Law of Definite Proportions
The law of definite proportions, also known as the law of constant composition, is a fundamental principle in chemistry that posits that a chemical compound will always contain exactly the same proportion of elements by mass. Consequently, the mass composition of a compound is invariant; that is, the ratio of the masses of the constituent elements in a compound is always constant, regardless of the sample's origin or preparation method. For instance, water (H2O) is composed of hydrogen and oxygen in a mass ratio of approximately 1:8, or 11.19% hydrogen and 88.81% oxygen.
This fundamental principle is significant in chemistry because it means the identity of a compound is defined by its elemental composition. It allows chemists to predict and understand the outcomes of chemical reactions and to quantify the reactants and products involved. The law ensures that chemical equations can be balanced and that stoichiometry can be applied to calculate the amounts of substances consumed and produced in chemical reactions.
Answering question #2, the composition of water molecules does not vary depending on its source because of the law of definite proportions. Whether it's tap water, seawater, or water produced in a laboratory, as long as it is pure H2O, it will contain the same proportions by mass of hydrogen and oxygen.
To summarize the importance, the law of definite proportions is crucial for the establishment of stoichiometry, consistency in chemical formulas, and the ability to replicate chemical reactions with predictability. It is one of the foundational laws on which the field of chemistry is built.