Answer :

Because you're turning moles into molecules, all you need to use is Avogadro's Number (6.022*10^23).
The equation would be: .400 mol N2O5/1 mol*6.022*10^23 molecules. 
The answer, including significant figures, is 151 molecules. 
transitionstate

Answer:

               2.40 × 10²³ Molecules of N₂O₅

Explanation:

As we know 1 mole of any substance contains 6.022 × 10²³ particles (also called as Avogadro's Number). Therefore, the relation between moles and number of particles can be written as,

             Moles  =  Number of Particles ÷ 6.022 ×10²³ Particles.mol⁻¹

For the molecules of Dinitrogen pentoxide (N₂O₅) it can be written as,

             Moles  =  Number of Molecules ÷ 6.022 ×10²³ Molecules.mol⁻¹

Rearranging for Number of Molecules,

             Number of Molecules  =  Moles × 6.022 ×10²³ Molecules.mol⁻¹

Putting value of Moles,

             Number of Molecules  =  0.400 mol × 6.022 ×10²³ Molecules.mol⁻¹

             Number of Molecules  =  2.40 × 10²³ Molecules of N₂O₅