Question

In a brief paragraph, describe how a salt can affect the pH of a solution. How do the Anions and Cations of the salt differently affect the pH?

How would the following salts affect the pH? Raise pH? Lower pH? No Effect?

1. CsCl (Cesium Chloride)
2. NH₄Br (Ammonium Bromide)
3. NaCN (Sodium Cyanide)
4. NH₄F (Ammonium Fluoride)

Answer 1

1) CsCl is a salt of strong acid, HCl and weak base, CsOH. Hence Cs+ and Cl- are weak conjugate acid and base respectively. This means no hydrolysis will occur and pH will not be affected.

2) NH4Br is a salt of strong acid, HBr and weak base, NH4OH. Hence ammonium ion will hydrolyse in water and pH will be lowered (see below).

3) NaCN is a salt of strong base, NaOH and weak acid, HCN. Hence cyanide ion (CN-) will hydrolyse in water and pH will be raised (see below).

4) NH4F is a salt of weak acid, HF and weak base, NH4OH. Hence both cation and anion will undergo hydrolysis. Now the exact effect on pH can be calculated using dissociation constants of HF and NH4OH. In such a case,

pH = 7 + 1/2(pKa - pKb)

Nhy (aq) + H₂O (1) Ź Nhy OH (aq) + 1 Ht (aq) lowered pH 3) ENCO ēN (aq) + H20 (1) ř HEN (aq) + (0h (aq)) raised pH NHỊ (19) + 10 (4) = NH4OH (aq) + H (aq) F" (aq) + Hold = HF (aq) + ÕH (aq?