Gchem question from Topscore

[healthcaree]

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When 14.25 moles of PCl5 gas is placed in a 3L container and comes to equilibrium at a constant temperature, 40% of the PCl5 decomposes according to the equation


PCl5 (g) <---> PCl3 (g) + Cl2 (g)

What is the value of Kc for this rxn?

A. (1.896)^2/2.854
B. (2.854)^2/1.898
C. (3.800)^2/2.854
D. (2.854)(1.896)/3.900
E. None of these



Answer is A. (1.896)^2/2.854



I just don't see where that 2.854 came from? isn't the concentration of PCl5 already calculated out as 4.750M ?

(14.25 moles/ 3L = 4.750M. 40% is reacting which means 4.750 x 0.40 = 1.896 M)

K(c) = ([PCl3]*[Cl2])/[PCl5]

I thought it was going to be

(1.896)^2/4.750.. but that wasn't an option, so i did pick A.. (whichever seemed the closest)

Can someone explain? Thanks!

 

[jsun4]

set up an ICE chart

PCl5 (g) <---> PCl3 (g) + Cl2 (g)
I) 4.75 =(14.25/3) 0 0
C) -1.9 +1.9 +1.9
E) 2.85 1.9 1.9

Kc=1.9^2/2.85

since 40% decomposed, the change in concentration is -1.9M

 

[healthcaree]

totally forgot about the ICE chart.. thanks again!!!

 

[jsun4]

🙂