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‘STRONG’ AND ‘WEAK’ ACIDS AND BASES 253

−
+
Equation (6.4) says K w = [H 3 O (aq) ] × [OH (aq) ], where the product of the two
−3 2
concentration has a value of 10 −14 (mol dm ) at 298 K. Knowing the values of K w and
+
[H 3 O (aq) ], we can calculate the concentration of hydroxide ions in the blood plasma.
Rearranging to make [OH (aq) ] the subject, we obtain
−
K w
−
[OH (aq) ] =
+
[H 3 O (aq) ]
Inserting values,
−3 2
10 −14 (mol dm )
−
[OH (aq) ] = −3
4 × 10 −8 mol dm
so
−
[OH (aq) ] = 2.5 × 10 −7 mol dm −3
Aqueous solutions in
We see how the pH of blood plasma is higher than 7, so the con-
centration of hydroxide ions exceeds their concentration in super- which the concen-
tration of hydroxide
pure water. We derive the generalization: aqueous solutions in
exceeds the concentra-
which the concentration of hydroxide is greater than the concen-
tion of solvated protons
tration of solvated protons have a pH higher than 7. The pH is show a pH higher
lower than 7 if the concentration of hydroxide is less than the than 7.
+
concentration [H 3 O (aq) ].
SAQ 6.7 A solution of ammonia in water has a pH of 9. Without using a
calculator, what is the concentration of solvated protons and hence what
is the concentration of hydroxide ions?

SAQ 6.8 What is the pH of sodium hydroxide solution of concentration
10 −2 mol dm −3 ?

6.2 ‘Strong’ and ‘weak’ acids and bases

Why is a nettle sting more painful than a burn
from ethanoic acid?

Introducing ‘strong’ and ‘weak’ acids

Brushing against a common nettle Urtica dioica can cause a pain-
ful sting. The active component in a nettle sting is methanoic acid The sting of a red ant
also contains meth-
(VI), also called ‘formic acid’. The sting of a nettle also contains
anoic acid.
natural additives to ensure that the methanoic acid stays on the
skin, thereby maximizing the damage to its sensitive underlying
tissue known as the epidermis.