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

Moderators: Chem_Mod, Chem_Admin

Chem_Mod
Posts: 17676
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 404 times

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

Postby Chem_Mod » Sun Aug 21, 2011 12:38 pm

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?

Chem_Mod
Posts: 17676
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 404 times

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

Postby Chem_Mod » Sun Aug 21, 2011 12:39 pm

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.


Return to “*Making Buffers & Calculating Buffer pH (Henderson-Hasselbalch Equation)”

Who is online

Users browsing this forum: No registered users and 1 guest