JEE Main On 08 April 2018 Question 23
Question: The predominant form of histamine present in human blood is $ \text{(p}{K_a},Histidine=6.0) $ [JEE Main Online 08-04-2018]
Options:
A) 
B) 
C) 
D) 
Show Answer
Answer:
Correct Answer: B
Solution:
Isoelectronic point $ =\frac{5.74+9.8}{2}=7.77 $
Histamine has two basic center which are Aliphatic amino group and imidazole ring.
In blood, the aliphatic amino group (pKa = 9.4) will be exist whereas the second nitrogen of imidazole ring (pKa = 5.8) will not be protonated in blood which pH is 7.4.
$ \text{pH=pKa+log}\frac{[\text{ Salt }]}{[\text{ Acid }]} $
$ 7\text{.4=}\underset{\begin{smallmatrix} \downarrow \\ \text{It should be high for balancing} \end{smallmatrix}}{\mathop{7\text{.77+log}\frac{[\text{ Salt }]}{[\text{ Acid }]}}}\ $
