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August 29, 2015

Homework Help: chemistry

Posted by Kelsy on Tuesday, April 8, 2008 at 1:45pm.

Hy Dr Bob222 this will be the last time I will bother you about this type of question. Can you please tell what I am doing wrong?
This is the best answer I could come up with:

Question: Strychnine, C21H22N2O2(aq) is a weak base but a powerful poison. Calculate the pH of a 0.001 mol/L solution of strychnine. The Kb of strychnine is 1.0 x 10^-6.

Calling strychnine SN,
SN + HOH --> SNH+ + OH-

Kb = (SNH+)(OH-)/(SN)
(SNH+) = x
(OH-) = x
(SN) = 0.001 - x

Plug Kb and solve for x = (OH-)

1.0 x 10^-6 = (x)(x)/0.001 - x
x^2 = 1.0 x 10^-9
x = 0.000031622
=-log (0.000031622)
=4.5

pH = 4.5

OR

x^2 + 0.001-x + 0.000001

Using quadratic formula gives

-0.001+0.0015 or -0.001-0.0015
= 0.0005 =negative answer

pH=-log^0.005
=2.3
pH = 2.30

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