Determine the mass fraction of aluminum in its oxide.

To determine the mass fraction of Aluminum (Al) in its oxide, you need to know the formula of aluminum oxide – Al2O3 And also the relative atomic mass of each constituent element and the molecular weight of the oxide.
From the periodic table we learn that
Ar (Al) = 26.98 amu
Ar (O) = 16 amu
Mr (Al2O3) = 26.98×2 + 16×3 = 101.96 amu
The mass fraction of the element (E) in the substance (EmOn) is determined by the formula: w (E) = mAr (E) / Mr (EmOn) х100%.
w (Al) = 3×26.98 / 101.96×100% = 79.38%
So, the mass fraction of aluminum in its oxide is 79.38%