How much water would be made using 2kg of hydrogen and 16kg of oxygen?

Molar mass of H₂ = 1 × 2 g/mol = 2 g/mol
Molar mass of O₂ = 16 × 2 g/mol = 32 g/mol
Molar mass of H₂O = 1×2 + 16 = 18 g/mol

Overall equation of the fuel cell : 2H₂ + O₂ → 2H₂O
OR: Mole ratio H₂ : O₂ = 2 : 1

Initial number of moles of H₂ = (2000 g) / (2 g/mol) = 1000 mol
Initial number of moles of O₂ = (16000 g) / (32 g/mol) = 500 mol
This fits the mole ratio H₂ : O₂ = 2 : 1
Therefore, both H₂ and O₂ completely reacts.

By the law of conservation of mass :
Mass of H₂O formed = (2 + 16) kg = 18 kg

First find out how many molecules of each element there are using the atoms molecular weight.
Then once you have the number of molecules, you can use waters structure H2O to find out how much of the reaction can go through until one of the elements gets completely used up.

18 kg. Mass, like, adds, dude.