unknown element X combines with oxygen to form the compund XO2. If 44.0 grams of element X combines with 8.00 grams of oxygen, what is the atomic mass of element X?
X + O2 ==> XO2
8.00 g x (1 mol O2/32 g O2) = 0.25 mol O2.
44.0 g element X must represent 0.25 mol element X.
Check my thinking.