Reaction
C₃H₄ + 4O₂ → 3CO₂ + 2H₂O
20% purity = 20% x 200 = 40 g
mole C₃H₄ = mass : molar mass = 40 : 40 g/mole = 1
mole H₂O from reaction coefficient (C₃H₄ limiting reactant) = 2/1 x 1 mole = 2 mole
mass H₂O = mole x molar mass = 2 x 18 g/mole = 36 g
for 60% yield, H₂O produced = 60% x 36 g = 21.6 g
