[AdiDex]

I've seen that Pka value of Acetic acid (CH₃COOH) is 4.3 while , ammonium ion NH₄⁺) have 9.3 .!!
That means accetic acid is better acid than Ammonium ion....!!
But in case of ammonium ion its conjugate base is neutral whereas in case of acetic acid it is an anion ..!!
 So ammonium should be more acidic ..!!

Is this due to solvation...?? That solvation helps it to gain stability ..!!
(Resonance also there )
But what will happen if i create an environment which is tends to be non-polar (Solvent) ...??

[Ben Bob2]

I have difficulty understanding some of your questions, but what I think you are asking is, "why is acetic acid more acidic than ammonium ion?"

In that case, you have to consider more than the charge of the conjugate base, namely you should consider inductive effects, resonance stability of the conjugate, electronegativity of A in A-H, and hybridization of A in A-H.

What is more elctronegative, oxygen or nitrogen?
Which of the two conjugate bases can form resonance structures? 
[I edited my spelling...]

[Borek]

But in case of ammonium ion its conjugate base is neutral whereas in case of acetic acid it is an anion ..!!
 So ammonium should be more acidic ..!!

First time I hear such a statement.

You mean HCl (with Cl^- being an anion) should be a weak acid? It isn't, so there is apparently something wrong with your logic.

[AdiDex]

Borek,
So what about  NH₂^-  and NH3 in term of basicity....??
I know that I'm wrong but i'm confused. 

[Borek]

You are overgeneralizing, that's your source of problems. You are taking into account one of the factors, when there are many that change.

Ben Bob2 already posted a list of things that have to be taken into account. I am not convinced his list is complete - but it is definitely a good starting point.

[AdiDex]

I have difficulty understanding some of your questions, but what I think you are asking is, "why is acetic acid more acidic than ammonium ion?"

In that case, you have to consider more than the charge of the conjugate base, namely you should consider inductive effects, resonance stability of the conjugate, electronegativity of A in A-H, and hybridization of A in A-H.

What is more elctronegative, oxygen or nitrogen?
Which of the two conjugate bases can form resonance structures? 
[I edited my spelling...]

Accefic acid's conjugate base can form resonatic structures and oxygen's electronegativity is more than Nitrogen .!

[AdiDex]

You are overgeneralizing, that's your source of problems. You are taking into account one of the factors, when there are many that change.

Ben Bob2 already posted a list of things that have to be taken into account. I am not convinced his list is complete - but it is definitely a good starting point.

So which point also should be added....??
How can we complete that list..??
I think charge density one of them..

[Ben Bob2]

Yes, you are correct about the charge density: when dealing with acids made from elements in the same group, the size must also be considered because larger atoms means lower charge density hence more stability of the conjugate.
I'm sure there are other criteria, but these five are the ones I am familiar with.