For the following reaction:

2NH3 ----> N2 + 3H3
6.42 moles of NH3 were placed in a 7.4 L flask and allowed to come to equilibrium. 3.60 moles of NH3 remain at equilibrium. Calculate Kc for the reaction.

3 answers

Set up an ICE chart, substitute into Kc and solve.
would you have to change the moles of NH3 to atm?
No. The question asks for Kc, not Kp. You may work in concn (molarity)from the beginning OR you may work in moles until the stoichiometry is finished, convert moles of each material to molarity and plug into the Kc expression.