Salts that consist of the cations of strong bases and the anions of strong acids have no effect on H+ when dissolved in water. In other words, aqueous solutions like NaCl, KCl, and NaNO3 are neutral, or have a pH = 7.

When cations of weak bases or anions of weak acids are dissolved water, though, pH is affected. An example is NaC2H3O2. Since C2H3O2- is the conjugate base of a weak acid, it has an affinity for protons. In water, it acts as a base to form OH- ions. For reactions such as this, the equilibrium expression is the same as for any acid or base, but sometimes, the Kb or Ka value is unknown. To find these values, we use the equation (Kb)(Ka) = Kw and Kw is equal to 1.0X10-14. So, to solve for the Kb of a conjugate base, you divide Kw by the Ka of the weak acid, or for a conjugate acid, you divide Kw by the Kb of the weak base

Ex. Find the Kb of C2H3O2-, the Ka of C2H3O2- is 1.8X10-5

(1.0X10-14) / (1.8X10-5) = 5.6X10-10

Remember that acids and bases are competing for H+ and OH- ions with water, so while the Ka of an acid, such as HCN is very small (6.2X10-10) and it would be expected to be a strong base, (1X10-14) / (6.2X10-10) = 1.6X10-5, which is still a weak base relative to other strong bases.

Base strength goes OH- > Weak Bases > H2O

Other salts produce acids in solution, such as NH4Cl. It dissociates to Cl- and NH4+, and NH4+ dissociates into H+ and NH3. Cl- has little to no affinity for protons in water, so it does not affect pH while ammonium does.

In general, salts in which the anion is not a base and the cation is the conjugate acid of a weak base produce acidic solutions.

Another salt that produces an acidic solution is one with a highly charged metal ion. An example is AlCl3, which produces a highly acidic solution in water. Al(H2O)63+ also functions as a weak acid, even though it is not a Br

Al(H2O)63+(aq) <=> Al(OH)(H2O)52+(aq) + H+(aq)

This is because the highly charged metal ions polarize the OH bonds in the water molecules, making the hydrogen in the water molecules more acidic than normal hydrogen in free water ions. Generally, the higher the metal ion charge, that stronger the acidity of the hydrated ion.

In a salt where both the anion and the cation affect pH, equilibrium expressions can become very complicated. To predict acidity of these solutions, the Ka and Kb values are compared. If the Ka value of the acidic ion is greater than the Kb value of the basic ion, the solution will be acidic while if the Kb value of the basic ion is greater than the Ka value of the acidic ion, the solution will be basic.

Next Section: The Effect of Structure on Acid-Base Properties

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