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The $pH$ value of gastric juice in human stomach is about 1.8 and in the small intestine it is about 7.8. The $pK_a$ value of aspirin is 3.5. Aspirin will be

(a) Ionised in the small intestine and almost unionised in the stomach.

(b) Ionised in the stomach and almost unionised in the small intestine.

(c) Completely ionised in the small intestine and in the stomach.

(d) Unionised in the small intestine and in tthe stomach.

1 Answer

Answer: Ionised in the small intestine and almost unionised in the stomach.
 
Given, $pK_a = 3.5 \Rightarrow log K_a = 3.5 \Rightarrow K_a = 10^{-3.5}$
 
Thus, $K_a$ of aspirin is quite small. As such it it a weak acid. In the presence of a storng acid is gastric juice ($pH = 1.8$) in stomach, it will be almost unionised. However, in small intestinw, where medium is slightly alkaline ($pH = 7.8$) it will be ionised.

 

answered Nov 28, 2013 by mosymeow_1
 

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