What mass of oxygen reacted with hydrogen of 0.4 g if water of 3.6 g was formed?

O2 + 2H2 = 2H2O.
1) Find the molar mass of hydrogen according to the periodic table: 1 * 2 = 2.
2) Find the amount of hydrogen substance: 0.4 / 2 = 0.2 mol.
3) Find the molar mass of water: 2 * 1 + 16 = 18.
4) Find the amount of water substance: 3.6 / 18 = 0.2 mol.
Substances in equal amounts. The calculation can be carried out for any of them. We’ll choose hydrogen.
5) The amount of oxygen is: 0.2 * 2 = 0.4 mol.
6) Determine the molar mass of oxygen: 16 * 2 = 32.
7) Find the mass of oxygen: 32 * 0.4 = 12.8 g – the answer.