equilibrium

5.00 mol of ammonia  are introduced into a 5.00 Liter reactor vessel in which it partially dissociates at high temperatures.
2 NHsubnumber3(g) = 3Hsubnumber2(g) +Nsubnumber2(g)
At equilibrium at a particulal temperature, 1.00 mole of ammonia remains. Calculate Ksubletterc for the reaction. Please show work  
Also briefly describe why addition of a catalyst does not affect the equilibrium constant for a reaction.

= is actually two arrows pointing opposite direction