what is the percentage aluminum in AICI3?

Mass of 1 mole of AlCl₃ = (27.0 + 35.5×3) g = 133.5 g
Mass of Al in 1 mole of AlCl₃ = 27.0 g

Percentage Al in AlCl₃ = (27.0/133.5) × 100% = 20.2%

percent aluminum.....

Simply use the molar masses.

26.98g / 133.33g x 100 = 20.24%

by weight/mass?

AlCl₃
atomic mass
Al = 26.98
Cl = 35.45

1 mole of AlCl₃ has 26.98 g of Al and 3•35.45 g of Cl for a total of 133.33 g
so the percentage of Al = 26.98 / 133.33 = 0.202 or 20.2 %

%Al = (molar mass Al / molar mass AlCl3) X 100
(27 g/mol Al / 133 g/mol AlCl3) X 100 = 20.3% Al