Question

A mixture of 1.71 moles of B and 1.23 moles of C is placed in a 1.00 liter container at a given temperature and allowed to reach equilibrium according to the following equation. Calculate [A] present at equilibrium. Calculate [A] with 3 significant figures, exponential notation, and no units.

2A <----> B + C     K_c= 3

Answer 1

molarity of B = 1.71 /1 = 1.71 M

molarity of C = 1.23 / 1 = 1.23 M

2 A <---------------------------->      B + C

0                                              1.71     1.23 ------------------> initial

2x                                             1.71-x   1.23-x -----------------> at equilibrium

Kc = [B][C]/[A]^2

Kc = (1.71-x) (1.23-x) / (2x)^2

3 = (1.71-x) (1.23-x) / (2x)^2

12 x^2 + 2.94 x -2.1033 = 0

by solving this

x = 0.3137

at equilibrium

[A] = 2x

[A] = 2 x 0.3137 = 0.6274 M

[A] present at equilibrium = 0.627 M ------------------------> answer