(1) Find the number of molecule in 1.6g of oxygen gas (l=6.023 x 1023)
Step I
Write formula of oxygen gas which is O2
Step II
Calculate the relative molecular mass of O2  gas
RMM of Oxygen =16
RMM of O2
=
+
=
Step III
Using the formulae
Number of moles of molecule =
    mass of molecule in g
             RMM
We substitute values,
Number of moles of O
=
           
     
=