Determine the mass fraction: a) nitrogen in the compound nitrogen oxide (V); b) oxygen in sodium nitrate.

a) M (N2O5) = Mr (N2O5) = Ar (N) * N (N) + Ar (O) * N (O) = 14 * 2 + 16 * 5 = 108 g / mol;
ω (N in N2O5) = N (N in N2O5) * Ar (N) * 100% / Mr (N2O5) = 2 * 14 * 100% / 108 = 25.93%.

b) M (NaNO3) = Mr (NaNO3) = Ar (Na) * N (Na) + Ar (N) * N (N) + Ar (O) * N (O) = 23 * 1 + 14 * 1 + 16 * 3 = 85 g / mol;
ω (O in NaNO3) = N (O in NaNO3) * Ar (O) * 100% / Mr (NaNO3) = 3 * 16 * 100% / 85 = 56.47%.