# 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?

From the equation you'll see that $2 {H}_{2} O$ gives $1 {O}_{2}$
This ratio ($2 : 1$) stays the same if we are talking about moles, because a mole is a set number of molecules ($\approx 6.02 \times {10}^{23}$). This goes for any substance.