Heavy water consists of deuterium instead of hydrogen . Deuterium atom consists one proton and one neutron. Hence, there are total 10 neutrons in a heavy water molecule (2 from deuterium and 8 from oxygen).
Number of heavy water molecules = 10/20 ${{N}{A}}$ =
0.5 ${{N}{A}}$
Mass of neutrons present in 0.5 mol of heavy water = 10/${{N}{A}}$ x
5 ${{N}{A}}$ = 5 g