Chemistry, Third edition

For example, the pKaof ethanoic acid at 25 °C is log(1.8 10 ^5 )4.75 and the

pKaof lactic acid is log(8.4 10 ^4 )3.08.

The stronger the acid, the lower is its pKa.

In equation (16.4), the acid is donating a hydrogen ion (a proton) to a water

molecule to produce a H 3 O(aq) ion. This means that we can look at a Kavalue as

an indicator of the proton-donating powerof that acid molecule toward the water

molecule:

The greater the Kavalue, the greater the proton donor ability of the acid

towards water.

The species A(aq) in equation (16.4) is referred to as the conjugate baseof the

acid AH(aq). This is because the A(aq) ion acts like a base in that it accepts a

proton from H 3 O(aq) in the reverse of reaction (16.4). For example, the conjugate

base of ethanoic acid is the ethanoate ion CH 3 COO(aq).

Calculating the pH of solutions of weak acids

We shall use ethanoic acid as an example of a weak acid which contains one acidic

hydrogen atom. We will symbolize the initial concentration of ethanoic acid in a

solution as CAmol dm^3. (For example, suppose we make a solution of ethanoic acid

by dissolving 0.1 mol of pure ethanoic acid in water and making the solution up to

1dm^3. Then CA0.1 mol dm^3 .) The difficulty in calculating the pH of the resulting

ACIDS AND BASES IN AQUEOUS SOLUTION 291

Table 16.2KaandKbvalues for selected weak acids and weak bases in

aqueous solution at 25 °C. These values change only slightly within the range

15–30 °C. The acidic hydrogen or basic nitrogen atom is highlighted in bold

Acids Ka/moldm^3

Trichloroethanoic acid CCl 3 CO 2 H 3.0 10 ^1
Chloroethanoic acid CH 2 ClCO 2 H 1.4 10 ^3
Sulfur dioxide SO 2 (sulfurous acid H 2 SO 3 ) 1.6  10 ^2
Lactic acid CH 3 CH(OH)CO 2 H 8.4 10 ^4
Hydrogen fluoride HF (hydrofluoric acid) 3.5  10 ^4
Benzoic acid C 6 H 5 COOH 6.5 10 ^5
Ethanoic acid CH 3 COOH 1.8 10 ^5
Carbon dioxide CO 2 (carbonic acid H 2 CO 3 ) 4.3  10 ^7
Hydrogen cyanide HCN (hydrocyanic acid) 4.9  10 ^10
Phenol (carbolic acid) C 6 H 5 OH 1.3 10 ^10

Bases Kb/moldm^3

UreaNH 2 CONH 2 * 1.3  10 ^14
Phenylamine (aniline) C 6 H 5 NH 2 4.3 10 ^10
Morphine C 17 H 19 O 3 N 1.6 10 ^6
AmmoniaNH 3 1.8 10 ^5
Methylamine CH 3 NH 2 3.6 10 ^4
Dimethylamine (CH 3 ) 2 NH 5.4  10 ^4
Triethylamine (C 2 H 5 ) 3 N 1.0 10 ^3

*KaorKbgiven for first ionization.