## Calculating acids,absorption and antibiotics?

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piloting
Garter
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Joined: Fri Dec 23, 2011 3:04 am

### Calculating acids,absorption and antibiotics?

There's a new antibiotic developed for oral admin

It has a pH=2 (stomach) , pH=6 in the intestine and pKa=2.1

a)Will the drug dissociate in aqueous soln's as a weak or strong acid?

b)Calculate the drug amount dissociated as % ionized and % non-ionized for each body part

c)Will the compound be absorbed by passive diffusion better in the intestines or stomach?

Here are some equations that'll help

pH=pKa+log[A-/HA]

Ka=[H+]*[A-]/[HA]

%[A-] ionized = ([A-]/[HA]+[A-]) * 100

%[HA-] non-ionized = ([HA-]/[HA]+[A-]) * 100

HA[two arrows pointing in opposite directions]A− + H+

pKa=-log10Ka

These questions have been a real stumper for lots of people in my class

Darby
Viper
Posts: 1278
Joined: Thu Mar 02, 2006 5:29 pm
Location: New York, USA

### Re: Calculating acids,absorption and antibiotics?

Do the environmental pH's (stomach and intestine) have to be factored in? I'm not sure, just asking.

piloting
Garter
Posts: 3
Joined: Fri Dec 23, 2011 3:04 am

### Re: Calculating acids,absorption and antibiotics?

Darby wrote:Do the environmental pH's (stomach and intestine) have to be factored in? I'm not sure, just asking.

Yes

JackBean
Inland Taipan
Posts: 5694
Joined: Mon Sep 14, 2009 7:12 pm
there is nothing like HA-! It's either A- or HA, as you have it in your equation HA = A- + H+

a) look up definitions of weak and strong acids.
b) all you need is pH=pKa+log[A-/HA]
c) which form (HA or A-) will be in stomach and which form will be in intestine? Which of these forms will difuse more easily through the plasma mebrane?
http://www.biolib.cz/en/main/

Cis or trans? That's what matters.

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