Equilibrium Concentration problem providing initial concentration of 2 reactants

Question

For the reaction below, Kc = 1.10 × 10⁻⁴. What is the equilibrium concentration of C if the reaction begins with 0.200 M A and 0.750 M B? 2 A (aq) + B (aq) ⇌ C (aq)

Answer 1

Set up an ICE table...

A(aq) + B(aq) ==> C(aq)

0.200.....0.750...........0.........Initial

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

0.2-x.....0.75-x...........x.........Equilibrium

Kc = 1.10x10^-4 = [C] / [A][B]

1.10x10^-4 = (x) / (0.20x)(0.75-x) and if we assume x is small we can ignore it in the denominator...

1.10x10^-4 = (x) / (0.2)(0.75) = x / 0.15

x = 1.65x10^-5 (which is small compared to 0.2 and to 0.75 so our assumption was valid)