Equilibrium and using ICE tables

Question

For the reaction below, Kc = 1.10 × 10⁻⁴. Note Kc is sometimes called K. What is the equilibrium concentration of C if the reaction begins with 0.200 M A and 0.400 M B? 2 A (aq) + B (aq) ⇌ C (aq)

Answer 1

2A (aq) + B (aq) ⇌ C (aq)

0.2........0.4.............0...........Initial

-2x.........-x............+x..........Change

0.2-2x.....0.4-x........x..........Equilibrium

Kc = 1.10x10^-4 = [C] / [A]²[B]

1.10x10^-4 = x / (0.2-2x)²(0.4-x) .. to simplify the math, assume x is small relative to 0.2 and ignore it

1.10x10^-4 = x / (0.2)²(0.4)

x = 1.76x10^-6 M (note: this is insignificant relative to 0.2, so above assumption was valid)

Equilibrium concentration of C = x = 1.76x10^-6 M