Hi
@Chelsea FC !
this question has been pored over previously on SDN so I will summarize what has been discussed.
Answer: B, To solve this question, use the formula X Pj = Ptotal- ln this case the following holds true:
Ptotal = PPCI3 + PC12 + pPCl5 = (1 " x) + (0-5 - x) + x = 1.5 - x.
If everything reacted to the limiting reagent capacity, then x is 0.5. But the reaction does not go to completion, so the value of x falls between 0 and 0.5. To solve this question, you must estimate the magnitude of x. The value of x is greater than 0.25, because over half shifts to products to reach an equilibrium constant of 1.95. This makes choice B the best answer.
You know that you start with 1.00 atm of PCl3 and 0.5 atm Cl2. Eliminate choice D immediately since you know total pressure will decrease as PCl5 is formed.
Total pressure at equilibrium = pressure of PCl3 + pressure of Cl2 + pressure PCl5 = (1-x) + (.5-x) + (0+x) = 1.5 - x
Note that x can't be greater than 0.5, since that means that Cl2 would have a negative partial pressure at equilibrium
So now you know that total pressure at equilibrium MUST be greater than 1.0 (since 1.5 - 0.5 = 1.0). Eliminate A.
How do you decide between choices B and C? If you are given that Keq = 1.95
Look at the case where x = .25. In that case, Q for the reaction will be (.25)/(.25)(.75) which is only 1.33. Since Q < K the reaction will keep shifting right. The answer must be B. Since Q < K, that means that the reaction needs to shift to the right. Since the reaction has to shift to the right, more product has to be made. For more product to be made, the x value has to increase and be bigger than 0.25. An increase in the x-value will decrease the expression "1.5 - x". This is why the answer is B between 1 to 1.25 atm.
hope this helps, good luck!