Who discovered water as h2o?

Answer 1

In the sense that water is two parts hydrogen and one part oxygen this was established by Gay-Lussac and von Humboldt.

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Answer 2

The chemical formula for water, H2O, was first proposed by Henry Cavendish in 1784. However, the discovery of the composition of water is credited to Antoine Lavoisier and Joseph Priestley in the late 18th century through their experiments on the decomposition of water into hydrogen and oxygen gases.