LoLPoPs

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Mar 31, 2008
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I'm struggling with TopScore GChem Question #56 in Exam #1.


When 14.250 moles of PCl5 gas is placed in a 3.00 liter container and comes to equilibrium at a constant temperature, 40% of the PCl5 decomposes according to the equation:

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


The answer is (1.896)^2/2.854


I can't figure out where 2.854 is coming from... I thought the bottom should be 4.75...??!? :confused::confused::confused:
Can someone please explain to me what's going on?
Thanks!!! :love:
 
May 15, 2009
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Dental Student
I'm struggling with TopScore GChem Question #56 in Exam #1.


When 14.250 moles of PCl5 gas is placed in a 3.00 liter container and comes to equilibrium at a constant temperature, 40% of the PCl5 decomposes according to the equation:

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


The answer is (1.896)^2/2.854


I can't figure out where 2.854 is coming from... I thought the bottom should be 4.75...??!? :confused::confused::confused:
Can someone please explain to me what's going on?
Thanks!!! :love:
I'm guessing they're asking for the equilibrium constant?

Start with 14.25 moles of PCl5, 40% decomposition means 14.25*0.4 = 5.7 moles decomposed. So, the remaining number of moles of PCl5 is 14.25-5.7 = 8.55 moles. Therefore, the equilibrium molar concentration of PCl5 equals:
M = n/L = 8.55/3 = 2.85M

At the same time, since you have a 1:1:1 ratio between all components of the reaction, 5.7 moles of PCl3 and Cl2 are produced as a result of 40% decomposition of PCl5. So, the molar concentration of these two species is 5.7/3 = 1.9 (Close enough to 1.869).

Now plug these #s into the formula for the equilibrium constant and that gives you the answer.
 
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LoLPoPs

LoLPoPs

10+ Year Member
7+ Year Member
Mar 31, 2008
110
0
Status
Dental Student
I'm guessing they're asking for the equilibrium constant?

Start with 14.25 moles of PCl5, 40% decomposition means 14.25*0.4 = 5.7 moles decomposed. So, the remaining number of moles of PCl5 is 14.25-5.7 = 8.55 moles. Therefore, the equilibrium molar concentration of PCl5 equals:
M = n/L = 8.55/3 = 2.85M

At the same time, since you have a 1:1:1 ratio between all components of the reaction, 5.7 moles of PCl3 and Cl2 are produced as a result of 40% decomposition of PCl5. So, the molar concentration of these two species is 5.7/3 = 1.9 (Close enough to 1.869).

Now plug these #s into the formula for the equilibrium constant and that gives you the answer.

Ohhhhhhhhhh I get it now.:D Thanks so much!!! :love: