PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 385

Solution

At a concentration as low as this, the activity coefficient of hydrogen ions is practically equal to

one. If, however, we used relation (11.86), we would obtain a nonsensical result

pH=−log(10−^8 ×1) = 8

implying that the solution of hydrochloric acid is basic. The error is caused by the neglect of the

concentration of hydrogen ions formed by the reaction (11.47). The correct procedure consists

in solving a chemical equilibrium. The concentration of hydrogen ions isc= 10−^8 +x, and the

concentration of OH−ions iscOH−=x, wherexis the concentration of the ions formed by the

dissociation of water. We substitute the concentrations into equation (11.49) and set the activity

coefficients equal to one

Kw= 10−^14 = (10−^8 +x)x =⇒ x= 9. 5 × 10 −^8.

From this we obtain the correct result

cH+= 10−^8 + 9. 5 × 10 −^8 = 1. 05 × 10 −^7 mol dm−^3 =⇒ pH= 6. 978.

11.6.5 pH of a strong monoacidic base

Provided that the concentration of the OH−ions formed by the dissociation of water is negli-
gible, it applies for a monoacidic base BOH thatcOH−=c, wherecis the base concentration.
The pH is determined by the relation

pH =−log

(

Kwcst

c γOH−

)

. (11.87)

11.6.6 pH of a strong dibasic acid and a strong diacidic base

For the pH of a strong dibasic acid H 2 A we have

pH =−log

c γH+

cst

−log (1 +α), (11.88)