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Base or Acid concentration

When doing an ICF chart or a ICE chart. How do I know my initial Base or Acid concentration does not need to be changed for the final amount? For example: 1.     (C2H5)3N + H2O -> (C2H5)3NH+ + OH-I .0304M 00C -x+x+xE .0304-xxx Kb = x2/.0304-x5.2×10-4 = x2/.0304-x0 = x2 + 5.2×10-4x – .0000158X= 5.2×10-4 +/- ((5.2×10-4)^2-(41-.0000158))^(1/2)/2X = -.00424, .0037M You can remove the negative answer.

Therefore [OH-] = [(C2H5)3NH+] = .0037[(C2H5)3N] = .0304-.0037 = .0267MpOH = -log [OH-] = -log(.0037) = 2.43pH = 14-2.43 = 11.57%dissociation = .0037/.0304 x100% = 12And # 2    C5H5N +H2O -> C5H5NH+ + OH-I .0143M00C -x+x+xE .0143-xxxKb = x2/.0143-x 1.7×10-9 = x2/.0143-x0 = x2+1.7×10-9x-2.431 x10-11X= -1.7×10-9 +/- ((1.7×10-9)2 – 4(1)(- 2.431 x10-11))1/2/2X = 4.93 x10-6 and – 4.93 x10-6. You can remove -4.93 x10-6 because a negative answer makes no sense.

Therefore [C5H5NH+] = [OH-] = 4.9 x10-6[C5H5N] doesn’t change so it is still .0143MThe pOH is easier to find pOH = -log(4.9 x10-6) = 5.31pH = 14-pOH = 14-5.31 = 8.69Why does C5H5N in #2 not change and (C2H5)3N does change for finding the concentrations?

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