Question

The number of molecules in 1.5 litres of water

Answer 1

1.5L water = 1500g 
Molar mass of H2O = 18 mol
Molar mass of  H2O in 1500g = 1500/18
                                                 = 83.33 mol 
1 Mole of H2O = 6.022*10²³ molecules 
Therefore 83.33 mol = 83.33*6.022*10²³
                                  = 5.0*10^25 molecules