How can I find the percent compositions of Al2(SO4)3?

To find the of Al₂(SO₄)₃, you divide the total mass of each atom by the molecular mass and multiply by 100 %.

% by mass = ##"mass of component"/"total mass"## × 100 %

Mass of 2 Al atoms = 2 Al atoms × ##(26.98"u")/(1"N atom")## = 53.96 u.

Mass of 3 S atoms = 3 S atoms × ##(32.06"u")/(1"S atom")## = 96.18 u

Mass of 12 O atoms = 12 O atoms × ##(16.00"u")/(1"S atom")## = 192.0 u

Mass of 1 Al₂(SO₄)₂ formula unit = (53.96 + 96.18 + 192.0) u = 342.1 u

% of Al = ##"mass of Al"/"total mass"## × 100 % = ##(53.96"u")/(342.1"u")## × 100 % =15.77 %

% of S = ##"mass of S"/"total mass"## × 100 % = ##(96.18"u")/(342.14"u")## × 100 % =28.11 %

% of O = ##"mass of O"/"total mass"## × 100 % = ##(192.0"u")/(342.14"u")## × 100 % =56.12 %