Indicate the number of oxygen atoms contained in one formula unit of ferum oxide, if the mass

Indicate the number of oxygen atoms contained in one formula unit of ferum oxide, if the mass fraction of iron atoms in it is 70%

Given:
FexOy
ω (Fe in FexOy) = 70%

Find:
N (Fe in FexOy) -?

Solution:
1) Let m (FexOy) = 100 g;
2) m (Fe in FexOy) = ω (Fe in FexOy) * m (FexOy) / 100% = 70% * 100/100% = 70 g;
3) n (Fe in FexOy) = m (Fe in FexOy) / M (Fe) = 70/56 = 1.25 mol;
4) m (O in FexOy) = m (FexOy) – m (Fe in FexOy) = 100 – 70 = 30 g;
5) n (O in FexOy) = m (O in FexOy) / M (O) = 30/16 = 1.875 mol;
6) x: y = n (Fe in FexOy): n (O in FexOy) = 1.25: 1.875 = 1: 1.5 = 2: 3;
Fe2O3;
N (O in Fe2O3) = y = 3.

Answer: The number of O atoms in Fe2O3 is 3.