Question #c8765

1 Answer
Stefan V.
May 23, 2015

The equilibrium concentration of D is 0.890 M.

The first part of the problem was already answered here, check it out:

http://socratic.org/questions/initially-only-a-and-b-are-present-each-at-2-00-m-what-is-the-final-concentratio?source=search

Now for the second part. This time, you start with 1.00 M of A and 2.00 M of B. Once again, use an ICE table to determine the equilibrium concentration of D.

" "A " "+ " "B " "rightleftharpoons " "C " "+" " D
I....1.00.............2.00...............0...................0
C....(-x)...............(-x)...............(+x)...............(+x)
E...1.00-x..........2.00-x...........x....................x

K_c = ([C] * [D])/([A] * [B]) = (x * x)/((1.00-x)*(2.00-x))

K_c = x^2/((1.00-x)(2.00-x)) = 6.5

Your cuadratic equation will look like this

5.5x^2 - 19.5x + 13 = 0

This equation will produce two solutions for x, 2.66 and 0.890. The first solution will be eliminated, since it implies that the equilibrium concentrations of A and B are negative.

This means that the equilibrium concentration of D will be

[D] = color(green)("0.890 M")

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