How many of each kind of atom is in one molecule of water?

a) 1 hydrogen atom and 1 oxygen atom

b) 2 hydrogen atoms and 2 oxygen atoms

c) 2 hydrogen atoms and 1 oxygen atom

The chemical formula for water is H2O which means that every molecule of water has 2 atoms of hydrogen (H) and one atom of oxygen (O). Here comes the key part. From the Periodic Table of Elements, one sees that one molecule of oxygen atoms weigh 16 grams.

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