Page 284 - Physical chemistry understanding our chemical world

PROPERTIES OF LOWRY–BRØNSTED ACIDS AND BASES 251

Worked Example 6.7 What is the pH of sulphuric acid having the same concentration
in water as the nitric acid in Worked Example 6.6?

This time, two solvated protons are formed per molecule of acid, so the concentration of
−3
[H (aq) ] will be 0.02 mol dm .
+
The pH of this acidic solution is obtained by inserting values into Equation (6.20):
pH =− log [2 × 0.01]
10
pH = 1.68

A pH electrode immersed in turn into these two solutions would
register a different pH despite the concentrations of the parent acids pH electrodes and pH
being the same. meters are discussed in
We need to be careful with these calculations, because the extent Chapter 7.
of dissociation may also differ; see p. 255ff.

What is the pH of a ‘neutral’ solution?
pH and neutrality

A medicine or skin lotion is often described as ‘pH neutral’ as though it was obvi-
ously a good thing. A solution is defined as neutral if it contains neither an excess
of solvated protons nor an excess of hydroxide ions. Equation (6.4) tells us the auto-
protolysis constant K w of super-pure water (water containing no
−3 2
additional solute) is 10 −14 (mol dm ) . Furthermore, we saw in
All neutral solutions
Worked Example 6.1 how the concentration of the solvated protons have a pH of 7 at
was 10 −7 mol dm −3 at 298 K.
298 K.
By considering both the definition of pH in Equation (6.20)
and the concentration of the solvated protons from Worked
Example 6.1, we see how a sample of super-pure water – which is
neutral – has a pH of 7 at 298 K. We now go further and say all The maximum pH of
neutral solutions have a pH of 7. By corollary, we need to appre- an acid will be just less
ciate how an acidic solution always has a pH less than 7. If the pH than 7, at 298 K.
is exactly 7, then the solution is neutral.
The pH of a Lowry–Brønsted acid decreases as its concentration The pH of a Lowry–
increases. Bench nitric acid of concentration 1 mol dm −3 has a Brønsted acid DEcre-
pH = 0. An acid of higher concentration will, therefore, have a ases as its concentra-
negative pH (the occasions when we need to employ such solutions tion INcreases.
are, thankfully, rare).

What do we mean when we say blood plasma
has a ‘pH of 7.4’?
The pH of alkaline solutions

Table 6.4 lists the pH of many natural substances, and suggests human blood plasma,
for example, should have a pH in the range 7.3–7.5. The pH of many natural