Question

How many H2 and O2 molecules do you need to make exactly 4 water molecules with no leftovers?

Answer 1

You need 4 H2 molecules and 2 O2 molecules.
This is because water is H2O which is 2 Hydrogen and 1 Oxygen so if you want to make 4 water molecules, you’ll need 8 hydrogen and 4 oxygen.
Given H2 you need 4 because 4•2 is 8 and for O2 you need 2 because 2•2 is 4.
I hope this makes sense.