Achieve #3 and calculating Ka and Kb
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Achieve #3 and calculating Ka and Kb
When calculating this problem after I have set up the equilibrium concentration equation to find x I approximated 8.4x10^4 as negligible (close enough to zero) because its smaller than 1x10^3. However I am getting the wrong answer doing it this way and am able to get the correct answer by solving with the quadratic equation. Achieve hints says the quadratic equation must be used because the percent ionization is greater than 5% meaning that the amount of lactic acid dissociated isn't negligible. Could someone please explain when and when not to approximate, and why the 5% ionization is significant?
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Re: Achieve #3 and calculating Ka and Kb
So whenever you test out and try to find percent ionization and it is greater than 5% you can no longer assume that x is 0 in the ICE table for the reactant. There is enough concentration being ionized that you cannot assume that the change is negligible. For this problem, youd have to redo it and find the concentration using quadratic formula because the percent ionization was 8.
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Re: Achieve #3 and calculating Ka and Kb
Yes, 8.40X10-4 does seem to be past our cutoff of 10-3. However, if you calculate the percent ionization, you get a percentage that is 5 or greater. At that point, you know that the approximation is not appropriate for this question because we have stated that 5% is our cutoff for approximations when checking our answer. Pretty much, we state that anything over 5% is no longer negligible, so you have to go back and not approximate anytime the percent ionization is greater than or equal to 5%.
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