drk1980
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Aspirin is a weak acid with a Pka of 3.5. At physiologic pH approximately what percentage of aspirin molecules are in the associated form?
A. 0.001%
B. 0.01%
C. 0.1%
D. 1%
E. 10%
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| msyamp
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PH = PKA + LOG (BASE/ACID)
7.5 = 3.5 + LOG (BASE/ ACID)
LOG BASE/ ACID = 4
LOG BASE/ACID =LOG 10000
BASE / ACID = 10000
BASE IS IONISED AND ACID IS UNIONISED
UNIONISED IS .0001
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| msyamp
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ANSWER IS NOT GIVEN
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| Doc2378
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Here I'd go with A Actually from my calculations it is = 1/ 1+ 10000
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| msyamp
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hey tricky one good one. it is .0001 that is .01% that is answer is B.
good one drk
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| Doc2378
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ooops....good one dude!
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| drk1980
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msyamp, good one alright! 
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| DrPak
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Actually, you don't need to calculate it... all those log calculations looks really scary. Anthony Trevor gave a much easier way. He said:
If the difference b/w the pH and pK is more than 2, then the the ratio (b/w ionized and non-ionized) will be 99.99:0.01.
If the difference is 2, then the ratio will be 99:1.
If the difference is 1 then the ratios will be 90:10.
If the difference is 0, then the ratio is 50:50.
In the question, the difference was >2, so immediately I thought that one form, either the ionized or non-ionized must be 99.99 and the other must be .01.
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| drk1980
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thts even better!
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| Geroo
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Drpak,so if the difference b/w PH and PK is 3 or 4 or 5,the ionized non ioniized ratio will be the same 99.99 : .01?
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| Doc2378
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Thats pretty useful DrPak...Thanks!
Also, if like me....the log scale scares you, you guys can try:
A-/HA = 10^(PH-Pka), where ^ denotes 'raised to the power'
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| DrPak
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The difference will not be much more than 2 because if it were you would be dealing with a strong acid or base, and we don't use such compounds in medicine. The physiological pH is always the same, its just the pK of the drug that changes. As you can imagine, if the difference was 5, then it means the pK of the drug must be something like 2 (if it's acidic) or 12 (if its basic). That's pretty strong stuff.
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| Geroo
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in this question the difference is almost 4.what if the the pk of the drug was 4.5 instead of 3.5.what is the ionized non ionized raio in this case using your rule?
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| DrPak
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The difference would still be more than 2. The answer would be the same.
Can someone calculate this to confirm?
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