Chemistry, Third edition

Acidic solution

An acidic solution is one where the concentration of hydronium ions is greater

than the concentration of hydroxide ions. At 25 °C, the pH of an acidic solution is less

than 7.

Basic solution

A basic (or alkaline) solution is one where the concentration of hydroxide ions is

greater than the concentration of hydronium ions. At 25 °C, the pH of a basic solution

is greater than 7.

Calculations using the ionic product constant of water

The ionic product constant of water (Kw) may be used to calculate the hydroxide ion

concentration in solutions of acids. It may also be used to calculate the hydronium

ion concentration in solutions of bases.

IONIC EQUILIBRIA IN WATER 287

Example 16.1

A 0.456 mol dm^3 solution of hydrochloric acid (HCl(aq))

contains 0.456 mol dm^3 of hydronium ions. Calculate the pH of

the solution of acid. What is the concentration of hydroxide ion

in the acid? (T25 °C)

Answer

First, calculate the pH:

pHlog (0.456) (0.341)0.341

Then, calculate the hydroxide ion concentration from Kw

[H 3 O(aq)[OH(aq)]1.0 10 ^14 mol^2 dm^6

or

[OH(aq)]

1.0 10 ^14



1.0 10 ^14

2.3 10 ^14 mol dm^3

[H 3 O(aq)] 0.436

Comment

The number of decimal places in the calculated pH equals to the number of

significant figures in [H 3 O(aq)], see Unit 1, page 13.

Calculations involving solutions of acids

(i)What is (a) the hydronium ion concentration and (b) the hydroxide ion concentration in a

0.10 mol dm^3 solution of nitric acid (HNO 3 )? What is the pH of this solution? (Nitric acid is

a strong acid and therefore completely dissociated into ions in solution.)

(ii)A solution of hydrochloric acid (a strong acid) has a pH of 2.20. What is the concentration

of hydronium ions in the solution?

Exercise 16B