Page 280 - Physical chemistry understanding our chemical world

PROPERTIES OF LOWRY–BRØNSTED ACIDS AND BASES 247

do not require us to cite the pH to more than 0.05 of a pH unit. In fact, we can rarely
cite pH to a greater precision than 0.01 for most biological applications.

Worked Example 6.2 What is the pH of bench nitric acid having a
−3
concentration of 0.25 mol dm ? The concentration of
the solvated protons in
Equation (6.18) needs
Inserting values into Equation (6.20):
to be expressed in the
−3 −3 familiar (but non-SI)
pH =− log[0.25 mol dm /mol dm ] = 0.6 −3
units of mol dm ;the
SI unit of concentration
The acid has a pH of 0.6. −3
is mol m .
SAQ 6.2 What is the pH of hydrochloric acid having a
concentration of 0.2mol dm −3 ?
We divide the concen-
SAQ 6.3 To highlight the point made above concerning tration by its units to
numbers of significant figures, determine the (negligible) yield a dimensionless
+
difference in [H 3 O ] between two acid solutions, one hav- number.
ing a pH of 6.31 and the other 6.32.
We are assuming the
Sometimes we know the pH and wish to know the concentration concentration of the
of the solvated protons. Hence, we need to rewrite Equation (6.20), proton H + is the same
making [H ] the subject, to obtain as the concentration of
+
+
H 3 O (aq) .
−3
[H /mol dm ] = 10 −pH (6.21)
+
Worked Example 6.3 What is the concentration of nitric acid having a pH of 3.5?
Inserting values into Equation (6.21):

+
[H ] = 10 −3.5
so
+
[H ] = 3.16 × 10 −4 mol dm −3
SAQ 6.4 What is the concentration of nitric acid of pH = 2.2?

Occasionally, we can merely look at a pH and say straightaway
Sometimes, we refer to
what is its concentration. If the pH is a whole number – call it
−3
x – the concentration will take the form 1 × 10 −x mol dm .As a whole number as an
integer.
−3
examples, if the pH is 6, the concentration is 10 −6 mol dm ;if
the concentration is 10 −3 mol dm −3 then the pH is 3, and so on.
Worked Example 6.4 Without using a calculator, what is the concentration of HNO 3
solution if its pH is 4?