Explanation:

The balanced chemical equation for the formation of water is:

$$2H_2 + O_2 -> 2H_2O$$

This equation tells us that 2 moles of hydrogen gas (H2) are required to react with 1 mole of oxygen gas (O2) to produce 2 moles of water (H2O).

We know that 36 grams of water is equivalent to 2 moles of water (since the molecular weight of water is 18 g/mol).

Therefore, to produce 2 moles of water, we will need 4 moles of hydrogen gas (since 2 moles of hydrogen are required for every 2 moles of water produced).

So the answer is 4 moles of hydrogen.