Number of molecules in 66.5 g N2O5

3.71 x10^23 molecules (corrected to 3 significant figures).

Explanation:

Take the atomic mass of N=14.0067; O = 15.999

no. of moles = mass / molar mass

no. of moles of N2O5 = 66.5 / (14.0067x2 + 15.999x5)

= 0.61569 mol

no. of molecules=  no. of moles x Avogadro's constant (6.022 × 10^23 mol-1)

Therefore,

no. of molescules=  0.61569 x 6.022 × 10^23

=3.71 x10^23 molecules (corrected to 3 significant figures).

the answer is c.the valence of the atoms combine .