[domique]

I cannot make out your most recent picture.  I would say that imidazole  exists primarily in the cationic state, but there is a significant fraction which is neutral.

So the answer to problem no.2 is imidazole is predominantly in cationic state, right ?

[Babcock_Hall]

Why predominantly in the cationic state and not the other way around?  Can you explain your reasoning to me?

[domique]

Why predominantly in the cationic state and not the other way around?  Can you explain your reasoning to me?

[Babcock_Hall]

It looks as if you used 7.6, not 7.4 as the pK_a in your calculations, but that is a minor error.  Everything else looks fine.  Problem 3) is a little trickier to handle quantitatively, but it can easily be dealt with by approximation or qualitatively.  The reasons for this are that the pK_a values are far apart and that the pH is closer to one than the other.

[domique]

Ok,thanks.
Yes, you're right I used the wrong number in calculating the pka ( my bad)