The balanced reaction is:
[tex]N_2+3H_2\rightarrow2NH_3[/tex]
According to this, 3 moles of H2 produce 2 moles of NH3. Use this ratio to find the number of moles of NH3 produced:
[tex]12molNH_3\cdot\frac{3molH_2}{2molNH_3}=18molH_2[/tex]
It means that 18 moles of H2 are needed to produce 12 moles of NH3
