Chemistry / Unit 2 Chem 0184 short lesson, about 20 seconds
The Law of Definite Proportions
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The idea
The law of definite proportions states every sample of a pure compound contains the same elements combined in the same fixed mass ratio.
In practice
Every sample of isooctane, wherever obtained, shows a carbon-to-hydrogen mass ratio of 5.33 to 1, regardless of the sample's source.
Where people slip
Sharing a mass ratio does not prove two samples are the same compound; other compounds can match that ratio.
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