Mass of Aluminum Sulfate: 2.74g
Molar Mass of Aluminum Sulfate: 342.14 g/mol
Therefore, number of moles of Oxygen can be calculated by 2.74g * 12/342.14 g/mol = 0.096 moles
Now since u have the moles of Oxygen, the number of oxygen atoms is given by
0.096 mol * (6.02*10^23) particles or atoms/mol
that gives u about 5.78 * 10^22 atoms of oxygen