As per laws of constant proportion/Proust's law,Ratio of elements by mass is always constant.

But in case of H2+O2 →H2O

Here when the chemists might have measured H2 and O2 individually they might have get a total mass of 2g+32g=34g And on measuring the product that is H2O they might get a mass of 2g+16g=18g

I know its not balance but still as you know whenever chemist will measure individually they will get the same mass as above.Hence,how the chemists mainly Proust might have defined this law?

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    $\begingroup$ You have silently dropped one oxygen atom in your chemical formula. That cannot work. $\endgroup$ – Karl Jul 4 '19 at 6:34
  • $\begingroup$ But Proust might have measured individually and might have got oxygen as 32g $\endgroup$ – user230507 Jul 4 '19 at 7:48
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    $\begingroup$ Yes of course! One mol of oxygen is 25 l at STP is 32 g. Two mol hydrogen is 50 l and 4 g. Together they become 2mol, 34 g of water, which take up 50l as a gas. $\endgroup$ – Karl Jul 4 '19 at 18:00
  • $\begingroup$ 2 mol of water are of course 36 g. $\endgroup$ – Karl Jul 8 '19 at 22:39

If you take the wrong amounts of each, you get not a pure product but some mixture with the educt!

The law of constant proportions says exactly that: Whenever you do that reaction with hydrogen and oxygen properly, you always get a volume ratio of (hydrogen:oxygen:water gas) 2:1:2, and a mass ratio of 1:8:9.

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Your chemical equation is not balanced, you just dropped one oxygen atom! First balance it! It should be:

$\ce{2 H2 + O2 -> 2 H2O}$

Then, you will get the correct masses.

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