Question

How many molecules of water and oxygen atoms are present in 0.9 gram of water?

Answer 1

Gram molar mass of water =18g 

Number of
moles in 0.9 grams of water 

$\frac{0.9}{18}$
= 0.05

Number of water molecules in 1 mole of water 

= $6.023 \times {10}^{23}$

Number of molecules of water in 0.05 moles 

= $0.05 \times 6.023 \times {10}^{23} = 3.010 \times {10}^{22}$

As one molecule of water contains one oxygen atom

So the number of oxygen atom in $3.010 \times {10}^{22}$

molecules of water = $3.010 \times {10}^{22}$