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:-
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