If 5 grams of hydrogen reacted with 40 games of oxygen to form water, how much of water would be formed?

2H₂ + O₂ = 2H₂O

n (H₂) = m (H₂) / M (H₂)

n (H₂) = 5g/2.0g/mol=2.5 mol

n (O₂) = m (O₂) / M (O₂)

n (O₂) = 40g/32.0g/mol=1.25 mol

H₂ : O₂ = 2 : 1

2.5 : 1.25 = 2 : 1

n (H₂O) = n (H₂) = 2n (O₂) = 2.5 mol

m (H₂O) = n (H₂O) M (H₂O)

m (H₂O) = 2.5mol*18.0g/mol=45.0 g