How many moles of nh3 from 19.5 mol of h2 and excess n2?

The balanced chemical equation is:
N2 + 3H2 → 2NH3

from above equation we can see that :
3 moles of hydrogen gas produce →2 moles of NH3.
=> 1 mole of hydrogen gas produce →2/3 moles of NH3.
=> 19.5 moles of hydrogen gas produce → (2/3)×19.5 moles of NH3.
hence moles of NH3 produced : 13 moles

hope it helps...

And a) oxygen i believe