First, we need to know this:
1 mol of Al2O3 = 101.96 g (Use your periodic table to get the molar mass of this compound)
For this particular exercise, we have 3 moles of the oxide, and we want to know how many grams the 3 moles represent.
Procedure:
1 mol of Al2O3 ----------- 101.96 g of Al2O3
3 moles of Al2O3 ----------- X
X = 305.88 g of Al2O3 = 306 g of Al2O3
Answer: The chemist needs to weigh out 306 g of Al2O3
