Answer
$ 1.0 \space mole \space O_2$ are required to react with 2.0 moles of $H_2$.
Work Step by Step
1. Determine the conversion factor between $O_2$ and $H_2$ moles:
According to the coefficients: 1 moles $O_2$ = 2 moles $H_2$.
$\frac{ 1 \space mole \space O_2}{ 2 \space moles \space H_2}$ and $\frac{ 2 \space moles \space H_2}{ 1 \space mole \space O_2}$
2. Use this conversion factor to calculate the amount of $O_2$ moles:
$2.0 \space moles \space H_2$ $\times$ $\frac{ 1 \space mole \space O_2}{ 2 \space moles \space H_2} = 1.0 \space mole \space O_2$