According to the Arrhenius concept, a base is a substance that produce OH - ions in aqueous solution. According to the Brønsted-Lowry model, a base is a proton acceptor. A strong base dissociates completely when dissolved in aqueous solution: NaOH(s) → Na + (aq) + OH - (aq) All the hydroxides of the Group I elements (LiOH, NaOH, KOH, RbOH, and CsOH) are strong bases. The alkaline earth (Group 2) hydroxides – Ca(OH) 2, Ba(OH) 2, and Sr(OH) 2 – are strong bases.

Many types of proton acceptors (bases) do not contain the hydroxide ion. However, when dissolved in water, these substances increase the concentration of hydroxide ions because of their reaction with water; thus, they yield a basic solution. Note below the ammonia molecule accepts a proton and thus functions as a base.

The general reaction between a base B and water is given by B(aq) + H 2 O(l) ⇌ BH + (aq) + OH - (aq) BaseAcid Conjugate acid Conjugate base The equilibrium constant for this general reaction is where K b always refers to the reaction of a base with water to form the conjugate acid and the hydroxide ion. Bases of the type represented by B are called weak bases.

pH calculations for solutions of weak bases are very similar to those for weak acids.

Some important acids, such as sulfuric acid (H 2 SO 4 ) and phosphoric acid (H 3 PO 4 ), can furnish more than one proton and are called polyprotic acids. A polyprotic acid always dissociates in a stepwise manner, one proton at a time. For example, the diprotic acid carbonic acid (H 2 CO 3 ), dissociates in the following steps: H 2 CO 3 (aq) ⇌ H + (aq) + HCO 3 - (aq)K a1 = 4.3 x HCO 3 (aq) ⇌ H + (aq) + CO 3 2- (aq)K a2 = 5.6 x

Typically, successive K a values are so much smaller than the first value that only the first dissociation step makes a significant contribution to the equilibrium concentration and the calculation of pH for a solution of a typical weak polyprotic acid is identical to that for a solution of a weak monoprotic acid. Sulfuric acid is unique in being a strong acid in its first dissociation step and a weak acid in its second step. For relatively concentrated solutions of sulfuric acid (1.0 M or higher), the large concentration of H + from the first dissociation step represses the second step, which can be neglected as a contributor of H + ions. For dilute solutions of sulfuric acid, the second step does make a significant contribution, and the quadratic equation must be used.

Salt is simply another name for ionic compound. When a salt dissolves in water, we assume it breaks up into ions, which are independent units. Under certain conditions, these ions can behave as acids or bases.

Salts that consist of the cations of strong bases and the anions of strong acids have no effect on [H + ] when dissolved in water. Aqueous solutions of salts such as KCl, NaCl, NaNO 3, and KNO 3 are neutral (pH = 7). For example, KCl(s) → K + (aq) + Cl - (aq) cation of strong base, KOH anion of strong acid, HCl

In an aqueous solution of sodium acetate (NaC 2 H 3 O 2 ), the major species are Na +,C 2 H 3 O 2 -,andH 2 O What are the acid-base properties of each component? The Na + ion has neither acid nor base properties. The C 2 H 3 O 2 - ion is the conjugate base of acetic acid, a weak acid. This means that C 2 H 3 O 2 - has a significant affinity for a proton and is a base. Water is a weakly amphoteric substance. The pH of this solution will be determined by the C 2 H 3 O 2 - ion, since it is a base.

In this solution, water is the only source of protons to react with the base, C 2 H 3 O 2 -, and the reaction is: C 2 H 3 O 2 - (aq) + H 2 O(l) ⇌ HC 2 H 3 O 2 (aq) + OH - (aq) Since OH - is produced, K b is defined as the equilibrium constant; therefore, How can we obtain the K b value for the acetate ion? The value of K a for acetic acid is known (1.8 x ) and can be used along with K w to obtain K b for the acetate ion.

For any weak acid and its conjugate base, K a x K b = K w Thus, when either K a or K b is known, the other can be calculated. For the acetate ion,

Some salts produce acidic solutions when dissolved in water. For example, when solid NH 4 Cl is dissolved in water, NH 4 + and Cl - ions are present, with NH 4 + behaving as a weak acid: NH 4 + (aq) ⇌ NH 3 (aq) + H + (aq) conjugateweak acidbase The Cl - ion does not affect the pH of the solution. 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.

A second type of salt that produces an acidic solution is one that contains a highly charged metal ion. For example, when solid aluminum chloride (AlCl 3 ) is dissolved in water, the resulting solution is highly acidic. Although the Al 3+ ion is not itself a Brønsted-Lowry acid, the hydrated ion Al(H 2 O) 6 3+ formed in water is a weak acid: Al(H 2 O) 6 3+ (aq) ⇌ Al(OH)(H 2 O) 5 2+ (aq) + H + (aq) Typically, the higher the charge on the metal ion, the stronger the acidity of the hydrated ion.

For many salts, such as ammonium acetate (NH 4 C 2 H 3 O 2 ), both ions can affect the pH of the aqueous solution. Compare K a and K b values for the acidic and basic ions and predict based on table below.