Asked by Mary

I think that titrating NH3 with HCl solution has a neutral equivalence point where the pH = 7.0. Is this right?
Answered by DrBob222
No, the equivalence point will be on the acid side of 7.0. Let me show you a very good way to know, it works every time, but it isn't very well accepted.
The REAL way is to know what are Bronsted-Lowry acids/bases.
The reaction is NH3(aq) + HCl(aq) ==> NH4Cl(aq) so the pH at the equivalence point is determined by the pH of the salt, NH4Cl.
So the REAL way is to know that NH4^+ is a stronger acid than H2O and the hydrolysis reaction is
NH4^+ + H2O ==> H3O^+ + NH3
and you can go through the acid/conjugate base thing. The H3O^+ tells you it is acidic.

Here is the way I know.
NH4Cl + HOH ==> NH4OH + HCl
NH4OH is a weak base (it has Kb = that of NH3). HCl is a strong acid. We know that. So the strong acid wins out over a weak base and the salt will be acidic.
Another example. sodium acetate
CH3COONa + HOH ==> CH3OOH + NaOH
Acetic acid, CH3COOH, is a weak acid. NaOH is a strong base so the salt will be basic (and it is when acetic acid is titrated with NaOH).
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