Which scientist’s law about the ratios of masses of elements in a compound did John Dalton’s work on the atomic structure help to explain?

Dalton’s atomic theory explained the law of definite proportions: a given compound always contains the same elements in the same fixed ratio by mass. That law was established by Joseph Proust, so the correct choice is Joseph Proust. The other scientists are associated with ideas about atoms or their structure, rather than this law of mass ratios.

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