Relationship between equilibrium constant of partial pressure and concentration

 

Consider a reversible reaction,

 

Where A, B, C, and D are reactants and products, m,n,p,q are coefficients. 

Using Le Chatelier principle, we can express the equilibrium constant of concentration and partial pressure as follows, 

Recall the ideal gas constant equation, 

but n/V is mol/L or molarity which is also a concentration (C), so we can rewrite the equation as, 

Now, we can substitute this to the equation of partial pressure equilibrium equation, 

Sample problem: 

Kc of equilibrium reaction below is 1.5x10^(-3) at 300 degrees C, what is K_p at this temperature? 

2NH3 --> N2+3H2

Solution: 

Δx=4-2=2