The molar mass of the water molecule is 18 g/mol. It implies that 1 mol of water molecules weigh 18 g.
Now you can calculate easily the number of water molecules present in 1 g of water. Calculation follows
18 g of water contains 1 mol of water molecules.
or 18 g of water contains 6.022 x 10^23 water molecules.
or 1 g of water contains (6.022 x 10^23)/ 18 water molecules and the number comes out to be 3.3455 x 10^22.
So the number of water molecules present in 1 g of water is 3.3455 x 10^22.
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Hope this helps you