How many grams of oxygen are contained in 8.50 g of Al2(SO4)3?

Answer 1

First, we find how many moles of Al₂(SO₄)₃ we have in 8.50 g:

molar mass Al₂(SO₄)₃ = 342.2 g / mol

8.50 g x 1 mol / 342.2 g = 0.02484 mols Al₂(SO₄)₃

Next we determine how many mols of oxygen (O) are in 0.02484 mols of Al₂(SO₄)₃:

0.02484 mols Al2(SO4)3 x 12 mols O / mol = 0.2981 mols O

Finally, we use the atomic mass of oxygen (O) to find grams of O:

0.2981 mols O x 16 g / mol = 4.77 g O (3 sig. figs.)

You could also solve this problem by finding the % by mass of O in Al₂(SO₄)₃ and then multiply that by 8.50 g.

% O by mass = 12 x 16 /342.2 = 0.561 (56.1% by mass)

56.1% x 8.50 g = 4.77 g O