J.R. S. answered • 03/10/19
Tutor
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B(aq) + H2O(l) ==> BH+(aq) + OH-(aq)
pH = 11.65
pOH = 14 - 11.65 = 2.35
[OH-] = 10-2.35 = 4.47x10-3 M
Kb = [BH+][OH-]/[B] = (4.4x10-3)(4.4x10-3)/0.033
Kb = 5.86x10-4
J.R. S.
tutor
That is always true, but I just did it the short-cut way to avoid using a quadratic equation. In this case, the [OH-] is greater than 5% of the 0.033, and so it should have been subtracted according to the "5% rule". So, now you can go ahead and subtract it, and you can use the quadratic.
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03/27/19
J.R. S.
tutor
Actually, it isn't the 0.033 that should be subtracted, but rather the 4.47x10^-3 that should be subtracted from the 0.033.
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03/27/19
Hunter R.
This is wrong, the correct answer is 7.0x10^-4, the .033 needs to be subtracted by the 4.47x10^-3 as well.03/27/19