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:
4x
2----------------- = .76
1.7-3x
.76(1.7-3x) = 4x
21.292-2.28 = 4x
24x
2+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?