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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.
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: