Chemistry, Third edition

ACIDS AND BASES IN AQUEOUS SOLUTION 289

Acids and bases in aqueous solution

Strong acids

Hydrochloric acid is made by dissolving hydrogen chloride gas in water. The dis-

solved HCl (symbolized HCl(aq)) then reacts with water as follows:

HCl(aq) H 2 O(l)H 3 O(aq)Cl(aq)

Hydrochloric acid consists almost entirely of hydronium and chloride ions and the

concentration of covalent acid molecules is negligible present. For this reason

hydrochloric acid is said to be a strong acid:

a strong acid is completely ionized in solution

This means that if 0.1 mol of HCl(g) is dissolved in water and made up to 1 dm^3 , the

concentration of both H 3 Oand Clions is also 0.1 mol dm^3.

There are only a few strong acids. Apart from HCl(aq), the list includes:

●perchloric acid (HClO 4 (aq)), and cheerfully referred to as the ‘strongest known

acid’ in TV quizzes;

●nitric acid (HNO 3 (aq)), whose ancient name, aqua fortis, means ‘strong water’;

●hydrobromic acid (HBr(aq)) and hydriodic acid (HI(aq)).

The case of sulfuric acid is discussed on page 294.

The term strong acidhas nothing to do with the corrosive properties of an acid. It

is also important to distinguish between acid strength (strong acid, weak acid) and

16.2

pH and pOH scale

The following table shows the hydronium and hydroxide ion

concentrations (in mol dm^3 ) for 14 aqueous solutions – complete the

table (the first entry has been done).

[H 3 O(aq)] [OH(aq)] pH pOH

10 ^141140

10 ^13

10 ^12

10 ^11

10 ^10

10 ^9

10 ^8

10 ^7

10 ^6

10 ^5

10 ^4

10 ^3

10 ^2

10 ^1

Exercise 16E