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according to my genius Teacher ' this law state that "the composition of a compound in terms of atoms (Not Mass) remains constant". His Proof: H2O and D2O are similar compounds (because H and D have same atomic number)...Now the story start from here---> The ratio by mass for H2O= 1:8, and for D2O= 1:4 (both have different ratios, although they are similar compounds- so This law shound not be defined on the basis of mass) Now, Ratio by atom for H2O= 2:1 and for D2O= 2:1 (both give same results, so this law should be defined on the basis of No.of atoms)...End of Proof Is the problem lie in similar compounds or my teacher is correct and the world is wrong?

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The Law of Definite Proportions was defined before we knew about isotopes. Sure, you can give a modern version of it, but that's not the original version.

If you have no notion of isotopes, then the mass is just a constant multiple (specifically, the atomic mass) of the number of atoms, in which case, the two definitions become equivalent.

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