## When can we neglect [x], Ka Kb value less than 10^-5?

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### When can we neglect [x], Ka Kb value less than 10^-5?

Question: When is it okay to approximate when finding the equilibrium concentrations of acids and bases by assuming that [x] is negligible? Is it when Ka or Kb is 10^-5 or less?

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### Re: when can we neglect [x], ka kb value less than 10^-5?

Answer: The whole Ka or Kb being 10^-5 or less is just a very general rule of thumb that tells you if you should even bother to assume the 5% rule (because if you do, and your x is NOT less than 5% of the initial concentration, you have to use the quadratic).
The approximation (a.k.a. the 5% rule) states that x must be less than 5% of the value we are comparing it to. For example, if the initial concentration is 0.25, 5% of 0.25 is 0.0125, and having an x value of 0.0095 ( less than 0.0125), we can use the approximation.