Answer:
2.408x10^23 molecules
Step-by-step explanation:
Avogadro's hypothesis gives us a background understanding that 1mole of any substance contains 6.02x10^23 molecules.
From the above conclusion,
1mole of N205 contains 6.02x10^23 molecules.
Therefore, 0.4mole of N2O5 will contain = 0.4 x 6.02x10^23 = 2.408x10^23 molecules