Page 283 - Physical chemistry understanding our chemical world

250 ACIDS AND BASES

Why do equimolar solutions of sulphuric acid
and nitric acid have different pHs?

Mono-, di- and tri-basic acids

Nitric acid, HNO 3 , readily dissolves in water, where it dissociates
Equimolar means ‘of according to
equal molarity’, so
+
equimolar solutions HNO 3(aq) + H 2 O −−→ 1H 3 O (aq) + NO − (6.27)
3(aq)
have the same concen-
tration. The stoichiometry illustrates how each formula unit generates a sin-
gle solvated proton. By contrast, sulphuric acid, H 2 SO 4 , dissociates
in solution according to

+
H 2 SO 4(aq) + 2H 2 O −−→ 2H 3 O (aq) + SO 2− (6.28)
4(aq)
so each formula unit of sulphuric acid generates two solvated protons. In other words,
each mole of nitric acid generates only 1 mol of solvated protons but each mole of
sulphuric acid generates 2 mol of solvated protons. We say nitric acid is a mono-protic
acid and sulphuric acid is a di-protic acid. Tri-protic acids are rare. Fully protonated
ethylene diamine tetra-acetic acid H 4 EDTA (V) is a tetra-protic acid.

OH O
O
OH
N CH 2 CH 2 N
O
OH
O OH
(V)

Equation (6.27) demonstrated how the concentration of the solvated protons equates
to the concentration of a mono-protic acid from which it derived; but, from Equation
(6.28), the concentration of the solvated protons will be twice the concentration if the
parent acid is di-protic. These different stoichiometries affect the pH, as demonstrated
now by Worked Examples 6.6 and 6.7.

Worked Example 6.6 Nitric acid of concentration 0.01 mol dm −3 is dissolved in water.
What is its pH?
+
Since one solvated proton is formed per molecule of acid, the concentration [H (aq) ]is
−3
also 0.01 mol dm .
The pH of this acidic solution is obtained by inserting values into Equation (6.20):
pH =− log [0.01]
10
pH = 2