How many grams of hydrogen are needed to get 72 g of water in the reaction?

Correct answer: c 8 g

72 g of water = 4 moles; 2H₂ → 2H₂O, so H₂ = 4 moles = 8 g.

Why the other options are wrong

• a 2 g: 2 g of H₂ gives only 18 g of water; 72 g of water needs 8 g.
• b 4 g: 4 g of H₂ gives 36 g of water; 72 g needs 8 g.
• d 16 g: 16 g is double, as if 8 moles of H₂ were used; 4 moles = 8 g.