How many grams of Aluminum are in 25.0 g of Aluminum oxide?

First you have to find how many moles of Al2O3 are in 25.0 g. To find that you divide the amount of grams by the molar mass.

molar mass = 2 (26.98) + 2 (16) = 85.96

Now to find the number of moles of Al2O3 you have ...

25 g / (85.96g / mol) =.291 moles (the grams cancelled out)

Now that ratio of the amount of Aluminum to Al2O3 is 2:1 (2 moles of Al for 1 mole of Al2O3)

so you much multiply the number of moles of Al2O3 by 2.

.291 (2) =.582 moles of Al

now Convert back to grams by multiplying the number of moles of Al by the molar mass of Al

so

.582 mol x 26.98 = 15.69 g