Many drugs are either weak acids or weak bases. Acetaminophen is a weak base, the acid form of which has pKa=9.5. Estimate the ration of (protonated/deprotonated) acetaminophen in each of the physiologic environment below: a. Stomach (pH ~ 2.0) b. Small intestine (pH ~ 8.0) c. Venous blood (pH ~ 7.35)

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Asked Dec 4, 2019
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Many drugs are either weak acids or weak bases. Acetaminophen is a weak base, the acid form of which has pKa=9.5. Estimate the ration of (protonated/deprotonated) acetaminophen in each of the physiologic environment below: 

a. Stomach (pH ~ 2.0) 

b. Small intestine (pH ~ 8.0) 

c. Venous blood (pH ~ 7.35) 

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Expert Answer

Step 1

The ratio of protonated to deprotonated ([HA]/[A-]) can be calculated by using the Henderson-Hasselbalch equation, which is given as,

[А ]
рH - pКа + log
нА
[HA]
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[А ] рH - pКа + log нА [HA]

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Step 2

Part A

Stomach (pH = 2)

The ratio can be calculated as,

рH - рКа + 1og нА
2 9.5+ log
[HA].
[A
[HA]
10-7.5
[A
— 3.16 х 10-8
HA]
HA]
3 3.16 х 107
[А"]
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рH - рКа + 1og нА 2 9.5+ log [HA]. [A [HA] 10-7.5 [A — 3.16 х 10-8 HA] HA] 3 3.16 х 107 [А"]

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Step 3

Part B

Small intestine (pH = 8)

The ratio can ...

[[А ]
рH - рКа + 1og нА
8 9.5+log
[гHA].
[А"]
HA]
10-1.5
[А]
0.0316
HA]
HA31.62
[НA]
[А"]
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Image Transcriptionclose

[[А ] рH - рКа + 1og нА 8 9.5+log [гHA]. [А"] HA] 10-1.5 [А] 0.0316 HA] HA31.62 [НA] [А"]

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