Chemistry, Third edition

Any solution which contains the hydroxide ion (such as NaOH(aq) or NH 3 (aq)) will

neutralize acids in the reaction:

H 3 O(aq)OH(aq)2H 2 O(l)

SelectedKbvalues are included in Table 16.2, with triethylamine being the strongest

base listed because it has the biggest Kbvalue. By coincidence, Kbfor ammonia (one

of the commonest weak bases) is numerically equal to Kafor ethanoic acid at 25C.

Books often tabulate pKbvalues where

pKblogKb

For example, the pKbof ammonia is log(1.8 10 ^5 )4.74. The stronger the

base, the loweris its pKb.

In reaction (16.8), the base is accepting a proton from a water molecule. This en-

ables us to use Kb(T)values as indicators of the proton-accepting abilityof bases in water,

in the same way that we used Ka(T)values as indicators of the proton donating ability

of acids in water. A large Kb(T)value shows that the base is a strong proton acceptor.

In equation (16.8), the species BH(aq) is referred to as the conjugate acidof

base B(aq). This is because BH(aq) donates a proton to the OH(aq) in the reverse

of reaction (16.8).

The self-ionization of water,

H 2 O(l)H 2 O(l)\===\H 3 O(aq) OH(aq)

involves one molecule of water donating a proton to another. This makes water both

a proton donor and a proton acceptor.

When calculating the pH of a weak base, we follow the same pattern as for an acid,

Kb(T)

[OH(aq)]^2

CB

whereCBis the initial molar concentration of base in the solution. If CBandKb(T)are

known, we can then calculate [OH(aq)]. [H 3 O(aq)] is then worked out using Kw.

(Now try Exercise 16H.)

HYDROLYSIS OF SALTS 295

Hydrolysis of salts

pH of solutions of salts

A salt is produced when a base and acid neutralize each other. On this basis, four

classes of salts are possible:

1.a salt of a strong acid and a strong base (SA–SB);

2.a salt of a weak acid and a strong base (WA–SB);

16.3

Calculations involving weak bases

(i) pKbfor nicotine, C 10 H 14 N 2 , is 5.98. What is Kb?

(ii)Estimate the pH of a solution of ammonia of concentration 0.200 mol dm^3 at 25 °C.

(iii)Write down an equation showing the reaction of phenylamine with water to produce

hydroxide ions. What is the conjugate acid of phenylamine? Some phenylamine was

dissolved in pure water. The pH of the solution was found to be 7.80 at 25 °C. What was

the concentration of phenylamine in the solution?

Exercise 16H