Question from text:

Question

"The reaction between phosphorus and chlorine is described by the equation, P4(s) + 6 Cl2(g) ↔ 4 PCl3(l). At equilibrium, the concentration of Cl2 is found to be 0.87 M. Calculate Keq."

Keq = [PCl3]^4 / [P4] [Cl2]^6

I am not seeing how to solve this when I don't have a value for Keq or more than 1 equilibrium value. I know there should be a way to calculate the other values through stoichiometric means, but I cannot seem to find the way!
I tried a straight molar ratio (out of frustration) and found:
0.87mol(Cl2)*1/6=0.145mol(P4)
0.87mol(Cl2)*4/6=.58mol(PCl3)
plugging that into the Keq eqiation:
Keq=[.58]^4/[0.145][0.87]^6
Keq=0.11316496/0.0628757991
Keq=1.7998
(Not the correct answer!)

Answer 1

I THINK your problem is that the value of 0.87 is two significant figures (s.f.) but your answer is 5 s.f. and that isn't kosher. Try rounding your answer to 2 s.f. (1.8) and see if that works.

Answer 2

I am having the same issue. I am wondering if the problem is set up wrong. I tried using ice table to figure it out, but still no luck

Answer 3

Actually it is neither of those responses. There is an exception to the equilibrium rule that is applied here. Solids and liquids are not involved in equilibrium problems. ONLY GASES. Since P4 and PCl3 are solids and liquids they are not considered in the Keq equation. You will only have Keq=1/[Cl2]^6. Plug in the values for 0.87 mol/L to solve your equation. So Keq=1/[.87]^6=1/0.434=2.304 That is your solution.