Question about disregarding the x when doing the ICE table

[Adrian Jones 3B]

So I know that when the Ka or Kb is less than 10^-5, we can disregard the x when solving for the quadratic. But can we still do that even if the K value is greater than 10^-5, like for example 3.6x10^-4 ? or do we have to include the x when solving for the quadratic?

[Niharika Reddy 1D]

If the K value is greater than 10^-5, you shouldn't approximate; leave the "-x" or "+x" in the equilibrium expression when solving for x.

If you are unsure whether or not to approximate, you can make the approximation by assuming the change (x) is negligible, but then going back and checking if x is less than 5% of the initial concentration. [(Calculated value of x with the approximation)/(initial concentration) x100%]. If x is less than 5% of the initial concentration (assuming the ICE box is in terms of concentrations), the approximation is valid.

[Claire Henderson 3J]

The 10^-5 is a guideline to use because it basically guarantees that the x will be 5% so you don't really have to check but if its something close like you mentioned you can do the approximation and check it. If it is more than 5 percent though, you'll need to go back and do it again without making the x negligible.