Chemistry 2 - Ice

Question

When determining the equilibrium concentrations for the reaction

AB(g)↽−−⇀A(g)+B(g)𝐾c=[A][B][AB]

a simplifying assumption can be used under certain conditions to avoid solving a quadratic equation.

𝐾c=𝑥2[AB]−𝑥≈𝑥2[AB]

Classify each situation by whether the simplifying assumption can be used or whether the quadratic formula is required.

Simplifying assumption can be used

Quadratic formula required

AB = 0.0140 M, Kc = 2.94 x 10^-6

AB = 0.00227 M, Kc = 2.69 x 10^-6

AB = 0.350 M, Kc = 6.20 x 10^-4

AB = 0.348 M, Kc = 2.00 x 10^-3

AB = 0.0481 M, Kc = 000116

Answer 1

Assumptions that are generally used to determine if you must use the quadratic include:

K value smaller than 10^-3

x less than 5% of the initial concentration

Using the first assumption, we would say that the simplifying assumption can be used for

AB = 0.0140 M, Kc = 2.94 x 10^-6

AB = 0.00227 M, Kc = 2.69 x 10^-6

AB = 0.350 M, Kc = 6.20 x 10^-4

AB = 0.0481 M, Kc = 000116

We'd also say that the quadratic would be required for

AB = 0.348 M, Kc = 2.00 x 10^-3