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ZingPath: Chemical Bonding

Law of Multiple Proportions

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Law of Multiple Proportions

Chemistry

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You will be introduced to the laws of definite proportions and multiple proportions by combining the ratios of five different gases.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Explain that the law of definite proportions states that a compound will have the same proportion of elements by mass.
  • Explain that the law of multiple proportions states that when two elements form more than one compound, the mass ratio of elements in one compound will be a small whole-number multiple of the mass ratio in another compound.
  • Determine the identity of a compound given the mass proportions of elements in the compound.
  • Compare two compounds that consist of the same elements to determine the mass ratio of elements in those compounds.
  • Explain that all compounds obey the laws of definite and multiple proportions.

Everything You'll Have Covered

Joseph Louis Proust formulated the law of definite proportions, which states that pure compounds always contain the same elements in the same proportion by mass. For example, always contains two moles of hydrogen and one mole of oxygen regardless of the size of the sample or source of the compound.

Building on Proust's work, John Dalton discovered the law of multiple proportions. When two elements combine, they combine in specific amounts. Atoms always combine with atoms in a whole number ratio. Partial atoms do not exist in compounds, which is why formulas do not have fractions such as .

The law of multiple proportions states that when two elements form more than one compound, the mass ratio of elements in one compound will be a small whole-number multiple of the mass ratio in another compound. For example, hydrogen and oxygen can react to form water, and hydrogen peroxide, . In the first compound, 2 grams of hydrogen reacts with 16 grams of oxygen in a 1:8 ratio. In the second compound, 2 grams of hydrogen reacts with 32 grams of oxygen in a 1:16 ratio. When the ratio of oxygen in each compound is compared the result is 32/16 = 2. This example demonstrates that the mass ratio of elements in the first compound is a small whole-number multiple of the mass ratio in the second compound.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Learners should be familiar with compounds, Dalton’s atomic model, elements, law of conservation of mass, molar mass, and percent composition.
Course Chemistry
Type of Tutorial Concept Development
Key Vocabulary atom, carbon, carbonate