The reaction referred to is N2(g) + 3H2(g) ===> 2NH3(g)
We will assume this reaction takes place at STP (standard temperature and pressure) since these variables aren't stated in the question. And the volume of 13 liters can represent a different number of moles depending on the temperature and the pressure.
At STP, 13 liters x 1 mole/22.4 liters = 0.58 moles of N2 gas
0.58 moles N2 x 2 moles NH3/ 1 mole N2 x 17 g NH3/mole NH3 = 19.7 g = 20. g of NH3 produced