Approximating Kc

[Nidhi Thati 3I]

In lecture, we learned that we can approximate in our calculations if the value of Kc is less than 10^-3. In the solutions for the textbook problems, they show the worked-out quadratic formula solutions for each problem, even if the value of Kc is 10^-4. Can we still make the approximation in this case?

Also, since there is no 5% ionization rule like there is when we are dealing with acids/bases, can we always approximate the equilibrium constant when it is less than 10^-3?

[Megan Efhan 3A]

Hi! With acids and bases, I believe you still use 5% ionization rule to approximate with acids and bases. Achieve says "if the value of x [dissociation]
is less than 5% of the solution concentration, the assumption that x is negligible is valid and the answer will be the percent ionization". In lecture, Dr. Lavelle said we can use the approximation rule when it applies so I'd assume that stands true even if the textbook works out all problems with the quadratic equation.