What is the percent composition by mass of aluminum in Al2 (SO4) 3 (gram-formula mass = 342 grams/mole) ?

(1) 7.89% (3) 20.8%

(2) 15.8% (4) 36.0%

The atomic mass of Aluminum is around 27 amu. There are two aluminum atoms, so it takes 54 amu. The gram-formula states that 1 mole of this compound has 342 grams. By definition, this means that the total amu of this compound is 342. Therefore, we divide 54 by 342 to get around 15.8 %.