Question

For each of the following salts indicate if it is acidic, neutral, or basic at 25°C: {1 point each} Given: C5H5N has a Kb = 1.7×10-9 , HF has a Ka = 7.2×10-4 , HBr is a strong acid, and KOH is a strong base KBr, KF, C5H5NHBr, and C5H5NHF l

Answer 1

There is three rules to predict the salt behaviour:

1. If both cation and anion are neutral, the salt is neutral.

2. If one of them is neutral, the another give the acid-base behaviour

2.1. If cation or anion is basic, and the another is neutral; the salt is basic.

2.2. If cation or anion is acidic, and the another is neutral; the salt is acidic.

3. If neither ion is neutral, we must compare their equilibrium constant.

Knowing that we can compare the salt given:

a) KBr (K is associated with a strong base; Br is associated with a strong acid)

K⁺ is neutral. OH^- is neutral. The salt is neutral.

b) KF (K is associated with a strong base; F is associated with a weak acid)

K⁺ is neutral. F^- is acidic. The salt is acidic.

c) C₅H₅NHBr (Br is associated with a strong acid; C₅H₅N is associated with a weak base)

C₅H₅N⁺ is neutral. Br^- is basic. The salt is basic.

d) C₅H₅NHF (F is associated with a weak acid; C₅H₅N is associated with a weak base)

Neither ion is neutral, here we have to compare Ka and Kb.

Ka > Kb The salt is acidic.