if you burned 3.0 g of magnesium and produced 5.0 g of product how much oxygen gas was used ?

Answer 1

2Mg + O2 ---> 2 MgO

Mg = 24.3 g/mol
O = 16 g/mol

so
48.6 grams Mg + 32 grams O2 --> 80.6 grams MgO

5/80.6 = .062 mol MgO produced

x/32 = .062
x = 1.985 grams O2

and we needed
y/48.6 = .062
y = 3.00 grams of Mg sure enough