# How were weights of elements and compounds determined? [duplicate]

I have browsed through many questions on this site but I still don't understand this:

How chemists in the past used to determine the weight of substances (which I believe they randomly called elements or compounds) without using modern techniques?

For example ... we know the molecular formula of water is $\ce{H2O}$

But how did chemists determine the composition of water?

How did they know hydrogen and oxygen, and not some other substance, made water and how could they determine the weight according to which combined?

What was the complete procedure behind it?

EDIT: Just like I said.... That answer still doesn't show us the complete picture... Moreover, the answerer has used phrases like (I don't know) and (Maybe) many times....

• – Mithoron May 30 '17 at 13:38
• @Mithoron...How did Chemists know they were burning Hydrogen and Oxygen and not something else? Moreover the answer in the duplicate question doesn't tell us How was the amount of Oxygen determined? That answer ,literally, doesn't tell us anything – user35508 May 30 '17 at 13:40
• @Mithoron Could you please remove the duplicate tag? I think you don't completely understand my point... – user35508 May 30 '17 at 13:45
• @Mithoron Imagine yourself in that era , Could you possibly explain how gases being burned made something liquid? Moreover can you even tell there were two gases? How would you isolate them? – user35508 May 30 '17 at 13:52
• @Mithoron At least please provide a reply or a reason why my question is not appropriate? – user35508 May 30 '17 at 14:01