Chapter 1: Problem 15
State and explain the law of definite proportions.
Short Answer
Expert verified
The Law of Definite Proportions states that a chemical compound always contains the same proportion of elements by mass regardless of sample size or source, exemplified by water always having a hydrogen-to-oxygen mass ratio of approximately 1:8.
Step by step solution
01
Understanding the Law of Definite Proportions
The Law of Definite Proportions, which was first put forth by Joseph Proust, states that a chemical compound always contains exactly the same proportion of elements by mass. This means that no matter the size or source of a particular pure sample of the compound, the proportionate relationship between the masses of the elements in the compound remains constant.
02
Illustrating with an Example
Consider water (H2O) as an example of the Law of Definite Proportions. No matter where water is obtained from, it always contains hydrogen and oxygen in a mass ratio of 1:8 (approximately), which correlates to the molar ratio of the elements in water, which is 2:1. Essentially, every 9 grams of water will be composed of 1 gram of hydrogen and 8 grams of oxygen.
03
Implications of the Law
This law is one of the foundational principles of chemistry, and it forms the basis for chemical formulae and stoichiometry. It implies that compounds are composed of elements in specific ratios and those ratios are consistent across all samples of that compound. It allows chemists to predictably work with different amounts of substances because they can rely on these constant proportions.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Chemical Compound Composition
Understanding the structure and make-up of chemical compounds is critical in chemistry. A chemical compound is a substance formed by the chemical combination of two or more elements in a fixed ratio. These proportions are tightly related to the concept of atoms and molecules. For example, water is always composed of two hydrogen atoms bonded to one oxygen atom, giving it a molecular formula of H2O.
When we look deeper, we discover that despite the number of molecules we have, the ratio of hydrogen to oxygen remains consistent. This forms the essence of the law of definite proportions—no matter the sample's quantity, the composition in terms of the ratio of elements stays constant. Thus, understanding the composition allows chemists to characterize and work with substances in a predictable and quantifiable manner.
When we look deeper, we discover that despite the number of molecules we have, the ratio of hydrogen to oxygen remains consistent. This forms the essence of the law of definite proportions—no matter the sample's quantity, the composition in terms of the ratio of elements stays constant. Thus, understanding the composition allows chemists to characterize and work with substances in a predictable and quantifiable manner.
Stoichiometry
Stoichiometry can be thought of as the recipe for chemical reactions. It involves the quantitative relationships between reactants and products in a chemical process. A stoichiometric calculation can tell you, for instance, how much of each reactant you need to use to create a desired amount of product.
Continuing with our water example, using the law of definite proportions, we can deduce that to form 18 grams of water, we need 2 grams of hydrogen and 16 grams of oxygen. Stoichiometry is used extensively in chemistry to ensure that reactions are carried out with the correct amount of substances, thus avoiding waste and optimizing efficiency. It is all about the balance and proportions of a chemical equation, which requires a clear understanding of the law of definite proportions.
Continuing with our water example, using the law of definite proportions, we can deduce that to form 18 grams of water, we need 2 grams of hydrogen and 16 grams of oxygen. Stoichiometry is used extensively in chemistry to ensure that reactions are carried out with the correct amount of substances, thus avoiding waste and optimizing efficiency. It is all about the balance and proportions of a chemical equation, which requires a clear understanding of the law of definite proportions.
Joseph Proust
The law of definite proportions is deeply rooted in historical chemistry and is attributed to Joseph Proust, a French chemist who formulated it between 1797 and 1804. His work was initially met with skepticism but was ultimately proven through rigorous experimentation. Proust's law states that chemical compounds are composed of a fixed ratio of elements, as dictated by their masses.
This concept was a significant departure from the prior belief that elements could combine in any proportion. Joseph Proust's work led to pivotal advances in chemical understanding, laying the groundwork for later scientists, including John Dalton, to develop atomic theory. Proust's insistence on the consistency of compound composition became a fundamental principle of modern chemistry.
This concept was a significant departure from the prior belief that elements could combine in any proportion. Joseph Proust's work led to pivotal advances in chemical understanding, laying the groundwork for later scientists, including John Dalton, to develop atomic theory. Proust's insistence on the consistency of compound composition became a fundamental principle of modern chemistry.
Mass Ratio of Elements
The mass ratio of elements in a compound is the cornerstone of the law of definite proportions. This ratio explains that in any sample of a pure compound, the masses of the constituent elements are always in the same proportions. This is also reflected in the compound's chemical formula which is derived from the relative masses of the elements involved.
To visualize this concept, imagine creating different batches of cookies with the exact same recipe. No matter how many cookies you bake, the ratio of flour to sugar, for instance, remains constant, ensuring the same taste and texture. Similarly, in chemistry, every sample of a pure compound contains elements in fixed ratios, thus maintaining consistent properties across all samples. This reinforces the predictability and uniformity that are essential in chemical reactions and compounds.
To visualize this concept, imagine creating different batches of cookies with the exact same recipe. No matter how many cookies you bake, the ratio of flour to sugar, for instance, remains constant, ensuring the same taste and texture. Similarly, in chemistry, every sample of a pure compound contains elements in fixed ratios, thus maintaining consistent properties across all samples. This reinforces the predictability and uniformity that are essential in chemical reactions and compounds.