PLEASE HELP WITH CHEM! my test is in a couple days!

[orangepopsicle]

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I'm having trouble understanding this problem:

When 14.520 moles of PCl5 is placed in a 3.00L container and comes to equilibrium at a constant temperature, 40.0% of the PCl5 decomposes according to the equation:

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

What is the value of Kc?

-I understand the fact that Kc will equal: [PCl3][Cl2]/[PCl5] and that the original PCl5 concentration will be 2.85 however I am confused by the fact that the [Pcl3][Cl2] is [1.9]^2. Shouldn't their concentrations be [1.4]^2 since the amount decomposed was 2.85?

 

[casper22]

The molarity = 14.25moles/3Lit = 4.75... If 40% of this decomposes (4.75*0.40 = 1.9) you are left with 2.85....

From the equation we know everything is 1 mole each... thus the concentration of PCl^3 and Cl^2 is 1.9 each...

I hope this helps...

 

[Lethstang]

nt

 

[casper22]

nt

U are correct bro...the answer is (1.9)^2/2.85

 

[orangepopsicle]

The answer is [1.9]^2/(2.854) and I think I am grasping the concept..

So the amount dissolved is always the same for all product concentrations if they are the same ratio? i thought that you would have to divide the amount dissolved by 2 since there are two different concentrations of products.

 

[casper22]

The answer is [1.9]^2/(2.854) and I think I am grasping the concept..

So the amount dissolved is always the same for all product concentrations if they are the same ratio? i thought that you would have to divide the amount dissolved by 2 since there are two different concentrations of products.

dont forget, U are comparing number of moles from the reactant to the product....