A salt is an ionic compound with a cation other than H⁺ and an anion other than OH^- or O₂^-. When a salt is added to water, it dissociates

NaCl(s) - > Na⁺(aq) + Cl^-

To determine if a solution of a salt is acidic or basic, consider the properties of the cation and anion that make up that salt.

Cations: Some cations act as weak acids, such as NH₄⁺ and metal cations. The cations that derive from strong bases do not react with water: this includes Li⁺, Na⁺ and the rest of the alkali metals as well as Ca⁺, Sr⁺² and Ba⁺².

Anions: Anions that are the conjugate bases of weak acids are basic. The anions derived from the strong acids, Cl^-, Br^-, I^-, NO₃^-, ClO₄^- and So₄^-2 do not react with water.

If both anion and cation react with water, then compare the K_a of the cation to the K_b of the anion. If K_a > K_b, the solution is acidic, if not, it is basic.

Example: Are solutions of NaCl, NH₄Cl, KF and NH₄NO₂ acidic or basic?

Solution: Compare the strengths of the cations and anions in the various salts

For NaCl, the cation is Na⁺. This does not react with water. The anion is Cl^-, this does not react either. Thus, a solution of NaCl is neutral.

For NH₄Cl, the cation is NH₄⁺ which is acidic. The anion is Cl^-, which does not react with water. Thus, a solution of NH₄Cl is acidic.

For KF, the cation is K⁺, which does not react with water. The anion is F^-, which is basic. Thus, a solution of KF is basic.

For NH₄NO₂ the cation is NH₄⁺ which is acidic. The anion is NO₂^- which is basic. We need to compare the relative strength of the anion and cation:

• NH₄^-, K_a = 5.6*10^-10
• NO₂^-, K_b = 1.7*10^-11

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