Relina , I don't know if I'll be of much help but here I go =]
I think it's because NH3 is a WEAK Base.
A : HCl and NaOH => since it's STRONG Acid then it has HIGH concentration of Hydrogen Ions.
B : HCl and NaCl => since it's STRONG Acid and hence has HIGH concentration of Hydrogen Ions.
C : NH
3 and it's salt =>
It's a WEAK Base so most likely to have LOWEST concentration of Hydrogen Ions!D : CH
3COOH and it's salt => has Low Hydrogen Ions since it's a WEAK Acid.
Bases have plenty of
OH- Ions where as Acids have plenty of
H+ Ions. In other words , the Base has fewer [H
+] than the Weak Acid =D
I just tried to make sense out of it so that I make myself understand it so I'm not 100 % sure but that's how I got it =]
I hope I helped