[nikita]

hi,
this is a question much like the one i posted earlier.  in my other question, Borek suggested i was rounding off my calculations mid-problem, so i stopped and tried this one without rounding until the end and i am still coming up with the wrong answer.  Please see if I am doing this right to begin with.  I am fairly certain that i understand what i am doing here.  i can understand if i am messing up the sig figs, but for me, i want to make sure i know the concept.

At 70K, CCl4 decomposes to carbon and chlorine.  The Kp for the decomp is .76.  Find the starting pressure that will produce a total pressure of 1.7 at equilibrium.

so,  i start with an equation-
         CCl4    C + 2Cl2
I:        y                   0     0
C:       -x                 +x   +2x
E:      y-x                  x      2x
---------------------------------------
x(2x)²
--------      =   .76
  y-x

to solve, i got rid of the y-x by replacing it with:
(y-x) + x +2x  =  1.7   (sum of partial pressures = 1.7atm) 
(y-x)= 1.7-3x
hence:
4x²
-----------------    = .76
   1.7-3x           

.76(1.7-3x) = 4x²
1.292-2.28 = 4x²
4x²+2.28x-1.292 = 0

solved quadratically, x=-0.92078691398927, x = 0.35078691398927

subbing x as .351 for the partial pressures:
(y-x) + x +2x  =  1.7
y-.351+.351 +2(.351) = 1.7
y+.702 = 1.7
1.7-.702= .998

substituting the positive x in to find y:
4x2
------  = .76
(y-x)

4(.35078691398927)2
------------------------  =.76
y-.35078691398927

.4922058361          =.76
---------------         
y-.35078691398927

.76y-.2665980546 = .4922058361
 .76y = .7588038907
y= .998

so the original concentration, y, should be .998 according to my calculations.  but alas it is wrong.  i tried the following:
Submitted   
   0.566   
   0.870   
   0.860   
   1.00   
   0.998
(even when i use the wrong sig figs, they take it anyway, so my actual number must be wrong)

help?   

[Astrokel]

x(2x)2
--------      =   .76
  y-x

to solve, i got rid of the y-x by replacing it with:
(y-x) + x +2x  =  1.7   (sum of partial pressures = 1.7atm) 
(y-x)= 1.7-3x
hence:
4x2
-----------------    = .76
   1.7-3x       

[nikita]

I meant to put:
x(2x)²
--------------------  =.76
y-x

and hence

4x²
-----------------
y-x

i originally put x for the C and 2x² for the 2Cl₂
i squared the entire 2x making it 4x²

i had 2x for the 2Cl²
because Kp = [C][Cl₂]²
                    -----------------------------------
                         [CCl₄]

is that wrong?  should it be 2x² and not 4x²?

[Astrokel]

x(2x)² = 4x²?

[nikita]

well, regardless of my math, C wasnt even part of the equation anyway.  I recalculated it and got 1.2 as an answer, but it was 1.3.  Oh well.  thanks for pointing out my mistake or i never would have realised that C wasnt included.  thanks for your help.