DrVirgo
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Example:
Morphine is a weak base (pKa 8.0). What percentage will be in the ionized form in the urine at a pH of 6.0?
Choices: 99, 90, 50, 10, 1.
This question is from Kaplan (i added the choices)
Anyway, I know how to do this problem looking at the table: Percentages Nonionized as a Function of pH....
pH - pKa = -2
-2 in the table corresponds to 1% Non-ionized, therefor the drug is 99% Ionized. (Answer: 99%)
My question is can you solve this problem using the formula for weak base:
ph-pKa = log [nonionized]/[ionized] ???
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| robin082006
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yes you can
for weak base: ph-pKa = log [nonionized]/[ionized]
for weak acid ph-pKa = log [ionized]/[nonionized]
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| DrVirgo
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yes, thats true,
But there are two unknowns here:
Unionized, and Ionized in this problem...
So how would one go about doing the problem based on the formula?
Can anyone please solve it using the formula with all steps??
Thanks.
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| robin082006
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We do not need to know nonionized and ionized. We just calculate the ratio between it.
For this q, example
weak base: ph-pKa = log [nonionized]/[ionized]
-2=log [nonionized]/[ionized]
----> nonionized/ionized =10 power -2 = 1/100, and we have non-ionized+ionized=1. Resolve these two equation, we have
non-ionized is nearly 1% and ionized is nearly 99%.
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| DrVirgo
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Got it.
Thanks Robin.
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