How many oxygen atoms are in 6.90 g of Al2 (SO4) 3? Express your answer using scientific notation with two decimal places.

First, you need to know the molar mass of the compound. The compound Aluminum Sulfate has a molar mass of 342.15 g/mol. From the subscript of the compound, there are 4*3 = 12 moles of oxygen in 1 mole of the compound. The solution is as follows:

6.90 g Al₂ (SO₄) ₃ * (1 mol/342.15 g) * (12 mol O/1 mol Al₂ (SO₄) ₃) * (6.022*10²³ atoms/mole) = 1.457*10²³ atoms of oxgyen