How many oxygen atoms are contained in 2.74 g of Al2(SO4)3?

Idon’t understand this page, can someone ?

What pressure will be exerted by 0.675 moles of a gas at 25*c if it is in a 0.750-l container?

Ahypothrticalax type of ceramic material is known to have a density of 2.10 g/cm3 and a unit cell of cubic symmetry with a cell edge length of 0.57 nm. the atomic weights of the a and x elements are 28.5and 30.0 g/mol, respectively. on the basis of this information, which of the following crystal structures is (are) possible for this material: sodium chloride, cesium chloride, or zinc blende