Answer
$ 5.0 \space moles \space H_2O$ are produced when 2.5 moles $O_2$ reacts.
Work Step by Step
1. Determine the conversion factor between $O_2$ and $H_2O$ moles:
According to the coefficients: 2 moles $H_2O$ = 1 mole $O_2$ .
$\frac{ 1 \space mole \space O_2}{ 2 \space moles \space H_2O}$ and $\frac{ 2 \space moles \space H_2O}{ 1 \space mole \space O_2}$
2. Use this conversion factor to calculate the amount of $H_2O$ moles:
$2.5 \space moles \space O_2$ $\times$ $\frac{ 2 \space moles \space H_2O}{ 1 \space mole \space O_2} = 5.0 \space moles \space H_2O$
