Does the approximation method not work when...

[Danielle_Cho_3A]

Does the approximation method not work when the problem implies that reactants will be formed?
For example, you have a case where the different substances are at equilibrium and then some amount of product is added. This puts the system out of equilibrium and so you have to do an ICE table where the "change" row has negative X values for the products. In this case, since the reverse reaction is favored, does this mean we have to look at the inverse of K to see if we can use the approximation method (where we ignore X in the expression # - X).

[Miyu Tang Dis3I]

The approximation method only works when Ka or Kb is 10^-5 or more. The number has to be very very small to assume that you can ignore the -x in the denominator. When you cannot assume to ignore the -x, you have to use the quadratic formula to solve for x in order to find the equilibrium concentration.

[Anna_Aldridge_2C]

Hi, I don't believe it matters if reactants/products are being formed as long as the K value is smaller than 10^-3. When I am unsure about the size of the K value, I like to be safe and use the quadratic formula method to solve for the values of x. Additionally, I think whether or not the inverse of K needs to be calculated depends on how the problem is presented. If it gives you the forward reaction and then asks for the reverse, then I suppose you would have to find the inverse (although I do not believe there has been a problem I have encountered like this yet). I hope this helps!