Question

Given the equation `2H_2O -> 2H_2 + O_2`, how many moles of `H_2O` be required to produce 2.5 moles of `O_2`?

Answer 1

From the equation you'll see that `2H_2O` gives `1O_2`

Explanation:

So every 2 molecules of water gives 1 molecule of oxygen.
This ratio (`2:1`) stays the same if we are talking about moles, because a mole is a set number of molecules (`~~6.02xx10^23`). This goes for any substance.
So to produce 2.5 moles of oxygen, you will need the double number of moles of water, i.e. 5 moles.