13. Acid-Base Properties of Salts

• Neutral Solutions

When dissolving NaCl in water, it breaks up into ions:

NaCl Na⁺ + Cl^-

The positive Na⁺-ion is an acid: conjugate acid of the base NaOH. This is a strong base: K_b >> 1. As K_a x K_b = 10^-14, this means that K_a for the Na⁺-ion is much smaller than 10^-14 and thus the Na⁺-ion is a much weaker acid than water. It will have no influence on the pH.

The negative Cl^--ion is a base: conjugate base of the acid HCl. This is a strong acid: K_a >> 1. As K_a x K_b = 10^-14, this means that K_b for the Cl^--ion is much smaller than 10^-14 and thus the Cl^--ion is a much weaker base than water. It will have no influence on the pH.

When dissolving NaCl in water, three particles are present: Na⁺ (acid) , Cl^- (base) and H₂O (amfolyt). The strongest acid is H₂O, the strongest base is also H₂O. The solution will be neutral.

A salt that consists of a cation of a strong base and an anion of a strong acid has no effect on pH when dissolved in water.

NaCl , KCl , CaCl₂ , BaCl₂ , NaNO₃ , KNO₃ , Ca(NO₃)₂ , Ba(NO₃)₂ , ...

• Acidic Solutions

When dissolving NH₄NO₃ in water, it breaks up into ions:

NH₄NO₃ NH₄⁺ + NO₃^-

The positive NH₄⁺-ion is an acid: conjugate acid of the base NH₃. This is a weak base: K_b = 1.8 x 10^-5. As K_a x K_b = 10^-14, this means that
K_a for the NH₄⁺-ion is equal to 5.6 x 10^-10 and thus the NH₄⁺-ion is a weak acid, but stronger than water. It will have an influence on the pH.

The negative NO₃^--ion is a base: conjugate base of the acid HNO₃. This is a strong acid: K_a >> 1. As K_a x K_b = 10^-14, this means that K_b for the NO₃^--ion is much smaller than 10^-14 and thus the NO₃^--ion is a much weaker base than water. It will have no influence on the pH.

When dissolving NH₄NO₃ in water, three particles are present: NH₄⁺ (acid) , NO₃^- (base) and H₂O (amfolyt). The strongest acid is NH₄⁺, the strongest base is H₂O. The solution will be acidic.

A salt that consists of a cation of a weak base and an anion of a strong acid produces an acidic solution when dissolved in water.

NH₄Cl , NH₄Br , NH₄I , NH₄NO₃ , ...

The same goes for salts from an amine and a strong acid. And also for salts that consists of an anion of a strong acid and a cation that either:

• has a high charge density (Al³⁺, Be²⁺ )
• acts as a lewis acid (ex. Fe³⁺, Cu²⁺, Zn²⁺ , ...)

Examples: Al(NO₃)₃, FeCl₃, ZnCl₂, Cu(NO₃)₂, ...

• Basic Solutions

When dissolving KOAc in water, it breaks up into ions:

KOAc K⁺ + OAc^-

The positive K⁺-ion is an acid: conjugate acid of the base KOH. This is a strong base: K_b >> 1. As K_a x K_b = 10^-14, this means that
K_a for the K⁺-ion is much smaller than 10^-14 and thus the K⁺-ion is a much weaker acid than water. It will have no influence on the pH.

The negative OAc^--ion is a base: conjugate base of the acid HOAc. This is a weak acid: K_a = 1.8 x 10^-5. As K_a x K_b = 10^-14, this means that
K_b for the OAc^--ion is equal to 5.6 x 10^-10 and thus the OAc^--ion is a weak base, but stronger than water. It will have an influence on the pH.

When dissolving KOAc in water, three particles are present: K⁺ (acid) , OAc^- (base) and H₂O (amfolyt). The strongest acid is H₂O, the strongest base is OAc^-. The solution will be basic.

A salt that consists of a cation of a strong base and an anion of a weak acid produces an basic solution when dissolved in water.

NaOAc , KOAc , NaF , KF , NaCN , KCN , ...

Summary

Acid-Base Properties of Salts

A salt that consists of a cation of a strong base and an anion of a strong acid has no effect on pH when dissolved in water. A salt that consists of a cation of a weak base and an anion of a strong acid produces an acidic solution when dissolved in water. A salt that consists of a cation of a strong base and an anion of a weak acid produces an basic solution when dissolved in water.