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Ch.2 - Atoms, Molecules & Ions
Chapter 2, Problem 77

What is the law of multiple proportions, and how does Dalton's atomic theory account for it?

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The law of multiple proportions states that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in the ratio of small whole numbers.
To understand this, consider two elements, A and B, forming two different compounds. If compound 1 has a mass ratio of A to B as m1:n1 and compound 2 has a mass ratio of A to B as m2:n2, then the ratio (m1/m2) to (n1/n2) will be a simple whole number.
Dalton's atomic theory explains this by proposing that elements are composed of indivisible atoms, and compounds are formed by the combination of atoms in fixed ratios.
According to Dalton, atoms of a given element are identical in mass and properties, and chemical reactions involve the rearrangement of these atoms.
Thus, the law of multiple proportions is explained by the fact that atoms combine in simple whole-number ratios, leading to the observed mass ratios in different compounds.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Law of Multiple Proportions

The law of multiple proportions states that when two elements form more than one compound, the ratios of the masses of one element that combine with a fixed mass of the other element can be expressed as simple whole numbers. This principle highlights the discrete nature of chemical compounds and supports the idea that elements combine in specific ratios to form different substances.
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Law of Multiple Proportions

Dalton's Atomic Theory

Dalton's atomic theory posits that matter is composed of indivisible atoms, which combine in fixed ratios to form compounds. This theory provides a framework for understanding chemical reactions and the formation of compounds, as it explains how different combinations of atoms lead to the creation of distinct substances, thereby supporting the law of multiple proportions.
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Atomic Mass and Ratios

Understanding atomic mass is crucial for applying the law of multiple proportions, as it allows for the calculation of mass ratios between elements in different compounds. By comparing the masses of the elements involved, one can demonstrate that the ratios of the masses of one element that combine with a fixed mass of another are whole numbers, reinforcing the principles outlined in Dalton's atomic theory.
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