Question

For the reaction A + B 2C, Kc = 1.7 x 10?3 at room temperature If the initial concentration of A = 2.24 M, B = 0.56 M, what is the equilibrium concentration of C? *Do you need to use the quadratic equation or can you assume that x is negligible?*

Answer 1

Given that;

initial concentration of A = 2.24 M, B = 0.56 M

and reaction A + B 2C, Kc = 1.7 x 10?3

A +               B    <----> 2C
2.24            0.56                  0    (initial)
-x                  -x                     2x (change)
2.24-x       0.56-x                2x   (at equilibrium)

Kc = [C]^2 / [A][B]
1.7*10^-3 = (2x)^2 / {(2.24-x) (0.56-x)}
since Kc is small, we can assume x is negligible then we can write

0.56-x= 0.56, therefore;

1.7*10^-3 = (2x)^2 / {(2.24) (0.56)}
x= 0.02 M

[C] = 2x = 2*0.02 = 0.04 M