PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 384

11.6.4 pH of a strong monobasic acid.

For a strong monobasic acid HA it applies thatcH+=c, where cis the acid concentration.
Then
pH =−log

c γH+

cst

. (11.86)

Example

Calculate the pH of a solution of HCl of the concentrationc= 3 mol dm−^3. The mean activity

coefficient at a given temperature and pressure isγ±= 1.316.

Solution

From relation (11.86) we approximate the activity coefficient of hydrogen ions using the mean

activity coefficient to obtain

pH=−log(3× 1. 316 /1) =− 0. 596.

Note: It is obvious from the result of the above example that the often heard assertion

about pH ranging in the interval from 0 to 14 is not true. Concentrated acids may have

their pH negative, concentrated bases may have it higher than 14!

Relation (11.86) applies on condition that the concentration of hydrogen ions formed by the
dissociation of water may be neglected. This assumption fails only in relation to the solutions
of acids (and bases) of an extremely low concentration.

Example

Calculate the pH of a solution of HCl of the concentrationc= 10−^8 mol dm−^3 at a temperature

of 298 K and the standard pressure.