Determine the mass to oxygen in the N2O5 compound

Given:
N2O5

To find:
ω (O in N2O5) -?

1) Using the periodic system of D.I.Mendeleev, calculate the relative molecular weight of N2O5:
Mr (N2O5) = Ar (N) * N (N) + Ar (O) * N (O) = 14 * 2 + 16 * 5 = 108;
2) Calculate the mass fraction of O in N2O5:
ω (O in N2O5) = N (O in N2O5) * Ar (O) * 100% / Mr (N2O5) = 5 * 16 * 100% / 108 = 74.07%.

Answer: Mass fraction of O in N2O5 is 74.07%.