calculate the number of molecules in 4g of oxygen.
Mass of oxygen (O2) = 4g
Molecular mass of oxygen = 32 g
Number of molecules of oxygen = Avogadro number x( mass of oxygen / molecular mass of oxygen)
Number of molecules = 6.023 X 1023 molecules x (4g / 32g) = 7.52X 1022molecules
so, 7.52X 1022molecules are present in 4g of oxygen.