306 16 · ACID–BASE EQUILIBRIA
16.1.Label the following as true (T) or false (F):
(i) The pH range of an indicator is the band of pH in which
the indicator is coloured.(ii)If methyl orange shows a yellow colour, this indicates that
the solution is basic.(iii)The pH of a solution of hydrochloric acid depends upon
its concentration.(iv)The acidity constant of an acid is the equilibrium constant
for its ionization in solution.(v)25 cm^3 of 0.02 mol dm^3 H 2 SO 4 (aq) requires 50 cm^3 of
0.02 mol dm^3 NaOH for neutralization.16.2.Calculate the pH at 25 °C of (i)0.080 mol dm^3 HNO 3 ,
(ii)0.080 mol dm^3 KOH
16.3.(i)Calculate the pH of water at 100 °C.
(ii) 11.0 cm^3 of 0.040 mol dm^3 sodium hydroxide and
9.0 cm^3 of 0.040 mol dm^3 HCl are mixed in a beaker. Cal-
culate the pH of the solution at 25 °C.16.4.(i)Write down an equation showing the ionization of the
(weak base) morphine (C 17 H 19 O 3 N) in water. Calculate
the pOH and pH of an aqueous solution containing 1. 0 mg
of morphine per dm^3 of water at 25 °C (Kb(morphine)
1.6 10 ^6 mol dm^3 ; ignore dissolved CO 2 ).(ii) What volume of 0.0010 mol dm^3 HCl would be required
to exactly neutralize 100 cm^3 of the morphine solution?
Write an equation for the neutralization reaction involved.16.5.The ionization of phenol (a weak acid) in water is repre-
sented by the equation:
C 6 H 5 OH(aq)H 2 O(l)\===\C 6 H 5 O(aq)H 3 O(aq)Estimate the pH at 25 °C of a solution of phenol of concentra-
tion 5.00 10 ^3 mol dm^3 (ignore dissolved CO 2 ).
16.6.The reaction of ammonia with water is represented by
the equation
NH 3 (aq)H 2 O(l)\===\NH 4 (aq)OH(aq)Show, by substitution, that the acidity constantof the conju-
gate acid NH 4 (aq), symbolized Ka(NH 4 (aq)), is related to
Kb(NH 3 (aq)) by the expression
Ka(NH 4 (aq))Kb(NH 3 (aq))KwCalculateKa(NH 4 (aq)) at 25 °C (Kb(NH 3 (aq) 1.8
10 ^5 mol dm^3 ).
16.7.(i)Solutions of carbon dioxide are weakly acidic:CO 2 (aq)2H 2 O(l)\===\HCO 3 (aq)H 3 O(aq)Derive the following expression for a solution containing dis-
solved CO 2 (g):Ka(T)[H 3 O(aq)]^2
[CO 2 (aq)](ii)Carbonated water (provided by home fizzy drink dis-
pensers) consists of water saturated with CO 2. Water satu-
ratedwith CO 2 at 20 °C contains 4.0 10 ^2 mol of
dissolved gas per dm^3 of solution. Calculate the pH of car-
bonated water at this temperature.16.8.Predict whether solutions of the following salts will be
basic, acidic or neutral: (i) phenylamine hydrochloride
(C 6 H 5 NH 4 ,Cl),(ii)ammonium ethanoate, (iii)ammonium
nitrate,(iv)potassium bromide, (v)sodium carbonate.16.9.0.100 g of benzoic acid, C 6 H 5 COOH, was added to
0.080 g of sodium benzoate salt (C 6 H 5 COONa), the mixture
was dissolved in water and made up to 100 cm^3. Estimate the
pH of the resulting buffer, and use equations to explain how
the buffer responds upon (i)the addition of acid and (ii)the
addition of alkali.16.10.The following figure shows the way the pH changes
when 0.100 mol dm^3 NH 3 (aq), a weak base, is titrated with
25.00 cm^3 of 0.100 mol dm^3 HCl(aq) at room temperature.
Briefly discuss the suitability of the indicators listed in Table
16.4 for this titration.16.11.Scientists have carried out experiments to see whether
athletes (for example, sprinters) might be able to improve their
performance by ingesting sodium hydrogencarbonate before
exercise. Explain the reasoning behind these experiments.Volume of 0.100 mol dm–3 NH 3 (aq) / cm^3004810pH10 20 30 40 5026Revision questions
DID YOU KNOW?
In 2004 scientists predicted that if CO 2 emissions continue at their current rate the average pH of the
oceans would fall from 8.2 to 7.7. This corresponds to a three-fold increase in [H+]. Check this calcula-
tion for yourself.