What is the % yield if 20 grams of hydrogen is reacted with 32 grams of oxygen and produces 9 grams of water?
H2 + O2 -> H2O

12.5%

Explanation:

32/16 = 2 moles of oxygen

2 x 18.01 x 2/1 = 72.04 g of H2O

9/72.04 x 100 = 12.5%

balanced equation: 2H2 + O2 → 2H2O

limiting reactant is oxygen