To solve this question, we need to calculate the amount of oxygen in Fe2O3 using the atomic mass of oxygen multiplied by the number of atoms of it.
So the atomic mass of oxygen is 16. There are 3 atoms of oxygen in Fe2O3. So the molar mass of oxygen in Fe2O3 is 16x3 = 48 g/mol. The molar mass of Fe2O3 is 160, which is 100% by mass. So we just need to do a rule of 3:
160 ---- 100%
48 ---- x
160x = 48 x 100
160x = 4800
x = 4800/160
x = 30%
Answer: The percent by mass of oxygen is 30%.
Alternative (2).