[Tesiacoil]

I can't for the life of me remember how to calculate the equilibrium concentrations of a specific species in a reaction when all I'm given is the temp, Kc, and the expression.

At 80 C , Kc= 1.87 x 10^-3 for the reaction

PH3BCl3(s)<--->PH3(g)+BCl3(g)

Now, I know the solid isn't included in the equilibrium expressions. So, anyone that remembers first year chemistry, I could really use your help. This is simple, I know it is, but my mind is going completely blank.

It says to calculate the eq concetrations of PH3 and BCl3 if a solid sample of PH3BCl3 is placed in a closed vessel and decomposes until equilibrium is reached,


Thankyou for anyone that can help. And I'm sorry if this is in the wrong forum. I'm desperate.

[Yggdrasil]

Well, the first step is to write the expression for the equilibrium constant:

K_c = [PH₃][BCl₃]

Where the [PH₃] denotes the concentration of PH₃ at equilibrium and the [BCl₃] denotes the concentration of BCl₃ at equilibrium.  As you correctly noted, the concentration of the solid is not included in the expression for K_c because the concentration of a solid is not going to change throughout the course of the reaction.  Now, all you need to do is to figure out a relationship between [PH₃] and [BCl₃], and then you'll have all the information you need to solve the problem.