CHM1 17 Salts Collection

When is a salt solution basic or acidic?

There are several guiding principles that summarize the outcome: 

1. Salts that are from strong bases and strong acids do not hydrolyze. The pH will remain neutral, at 7. Halides and alkaline metals dissociate and do not affect the H⁺ as the cation does not alter the H⁺ and the anion does not attract the H⁺ from water. This is why NaCl is a neutral salt. In General: Salts containing halides (except F^-) and an alkaline metal (except Be²⁺) will dissociate into spectator ions.
2. Salts that are from strong bases and weak acids do hydrolyze giving it the pH greater than 7. The anion in the salt is derived from a weak acid, most likely organic, and will accept the proton from the water in the reaction having the water act as an acid in this case leaving a hydroxide ion (OH^-). The cation will be from a strong base, meaning from either the alkaline or alkaline earth metals so like before it will dissociate into an ion and not affect the H⁺. 
3. Salts of weak bases and strong acids do hydrolyze giving it a pH less than 7. This is due to the fact that the anion will become a spectator ion and fail to attract the H⁺ ion, while the cation from the weak base will donate a proton to the water forming a hydronium ion.
4. Salts from a weak base and weak acid also hydrolyze as the others, but a bit more complex and will require the K_a and K_b to be taken into account. Whichever is the stronger acid or weak will be the dominate factor in determining whether it is acidic or basic. The cation will be the acid, and the anion will be the base and will form either form a hydronium ion or a hydroxide ion depending on which ion reacts more readily with the water.

Summary of Acid Base Properties of Salts

Type of Solution	Anions	pH
Acidic	From weak bases NH4+, Al3+, Fe3+	From strong acids Cl-, Br-, I-, NO3-, ClO4-	< 7
Basic	From strong bases Group 1A and Group 2A, but not Be2+	From weak acids F-, NO2-, CN-, CH3COO-	> 7
Neutral	From strong bases Group 1A and Group 2A, but not Be2+.	From strong acids Cl-, Br-, I-, NO3-, ClO4-	=7

Example 6

Predict whether aqueous solutions of these compounds are acidic, basic, or neutral.

1. KNO₃
2. CrBr₃·6H₂O
3. Na₂SO₄

Worked Problem Here