Acids and Bases: Summary.

To find the pH of a given compound:

1. Decide if the compound is an acid, a base or a salt or if the solution contains conjugates and so is a buffer.

2. Acids.

Acids (Brønsted) have the general formula HA since they must transfer a proton to base.

Strong acids are:

HCl, HBr, HI, HNO3, H2SO4, HClO4.

Strong acids are fully ionized and their pH is found by stoichiometry.

Weak acids are: anything else.

Weak acids are partially ionized and require the use of equilibrium problem solving techniques (ICE, Ka).

Ka = {[H3O+] * [A-]} / {[HA]} = x * x / (a - x)

3 Bases.

Bases must have a lone pair to accept a proton from the acid, and often have nitrogen: NR3 (where R is either H or an organic group). E. g. NH3, C3H7NH2, (C2H5)3N.

Strong bases are the metal hydroxides, MOH.

Strong bases are fully ionized and the [OH-] can be found by stoichiometry. pOH can be found from this, and hence pH.

Weak bases are anything else.

Weak bases are only partially reacted with water and so must use equilibrium problem solving techniques (ICE, Kb).

Kb = {[BH+] * [OH-]} / {[B]} = x * x / (b - x)

4 Salts.

Salts are ionic products of acid base reactions and so contain conjugates of the acid and base they can be derived from. Salts are fully ionized in water, and they can be divided into acid, base, and neutral salts.

Acid salts contain the conjugate acid of a weak base plus a neutral anion. They are treated as weak bases (see above) using equilibrium problem solving techniques (ICE, Ka of the cation). If Ka of the cation is not known, it may be calculated from the Kb of the conjugate base, since Ka * Kb = Kw (for conjugates).

Basic salts contain the conjugate base of a weak acid plus a neutral cation. They are treated as weak acids (see above) using equilibrium problem solving techniques (ICE, Kb of the anion). If Kb of the anion is not known, it may be calculated from the Ka of the conjugate acid, since

Ka * Kb = Kw (for conjugates).

Neutral salts contain a neutral cation and a neutral anion. Since neither ion reacts with water, these salts have a pH of 7 when dissolved in water.

5 Solution contains a pair of conjugates, a buffer solution.

a Acid plus conjugate anion.

If the acid is strong, the solution is not a buffer and the pH can be calculated from the acid concentration alone.

If the acid is weak, then use Ka or pKa with the concentrations of the acid and its conjugate anion (which may be in the form of a fully dissociated salt).

pKa = pH - log {[A-] / [HA]}

b Base plus conjugate cation.

If the base is strong, the solution is not a buffer and the pOH can be calculated directly from the base concentration alone.

If the base is weak, then use Kb or pKb with the concentrations of the base and its conjugate cation (which may be in the form of a fully dissociated salt).

pKb = pOH - log {[BH+] / [B]}