1 molecule of H2O has 1 atom of oxygen.

Firstly we will calculate the number of atoms in 90g of H2O.

we will explain it in two steps:-

Step I:-

No.of moles of H2O = given mass / molar mass

No. of moles = 90/18 = 5 moles

No. Of molecules = No. of moles x Avogadro number

=5 x 6.022×1023

Step II :-

1 molecule has = 1 oxygen atom

So the number of atoms of oxygen = number of molecules of water

No. of oxygen atoms = number of H2O molecules

No. of oxygen atoms = 30.11 x 1023

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