Calculating Nitrogen Conversion to Ammonia at 773K Using the Law of Mass Action

[physics_newbi]

A mixture of one part nitrogen and three parts hydrogen is heated to a temperature of 773 Kelvin. Use the law of mass action to calculate the fraction of nitrogen that is converted (atom to atom) to ammonia if the final total pressure is 400 bar. As alwyas, use 1 bar for the reference pressure, and assume for simplicity that the gases behave ideally. The equilibrium constant K = exp(-dG/RT) at 773 K is 6.9X10-5.

Can someone please give me a hint on how to approach this problem

 

[HallsofIvy]

The best 'approach' is to first right down the "law of mass action'! Exactly what does it say?