TEST
BUFFER SOLUTIONS
Section A
1 For the
reaction HCl + H2O ó H3O+
+ Cl-, H2O
acts as the
A conjugate acid of H3O+
B conjugate acid of Cl-
C conjugate base of H3O+
D conjugate base of Cl-
2. Which of the following is an appropriate set of units for expressing the equilibrium constant of the equilibrium
HCrO4(aq)
ó H+(aq) + CrO4-(aq)
A mol dm-3
B mol-1 dm-3
C mol-1 dm3
D mol-2 dm6
3.
Which one of the following has the lowest pKa
value in aqueous solution at
298 K?
A chloroethanoic acid
B ethanoic acid
C water
D sodium hydroxide
4. HY is an acid which dissociates
completely in water to form H+(aq) and Y-(aq). Which one of the following is the
concentration, in mol dm-3 of an aqueous solution of this acid which
has a pH of 2?
A 0.2
B 0.1
C 0.02
D 0.01
5. The value of pKa of
trichloroethanoic acid is 0.7 at 298 K. Which one of the following statements
about 0.01 mol dm3 trichloroethanoic acid at 298 K is correct?
A The concentration of hydrogen ions is
2.3 x 10-4 mol dm-3
B The concentration of hydroxide ions is 10-13.3
mol dm-3
C The pH of the solution is 2.3
D The numerical value of Ka is 10-0.7
6. Which one of the following statements about a chemical equilibrium at a
given
temperature is incorrect?
A The
composition of the equilibrium mixture is independent of the direction from
which the equilibrium is established.
B If the
initial concentration of one or more of the reactants is increased, the
equilibrium constant will be unchanged.
C The rate
of the forward reaction is the same as that of the reverse.
D The
composition of the equilibrium mixture may be altered by the addition of a
suitable catalyst.
7. Which of the following statements
about an indicator is false?
A The
conjugate base is In-(aq)
B The
colour of the indicator depends on the relative concentrations
of In-(aq) and HIn(aq).
C The
indicator changes colour when the concentrations of In-(aq)
and HIn(aq) are equal
D Addition of alkali would reduce the
concentration of In-(aq) at
equilibrium.
8 The value of Ka for an acid HX is 1.6 x 10-1 mol dm-3 at 298 K.
Which of the following is the hydrogen ion concentration in mol dm-3 in a 0.1 mol dm-3 solution of HX at this temperature?
A 1.6 x 10-5
B 1.6 x 10-3
C
4.0 x 10-3
D
4.6 x 10-2
Section B
1
(a) Define the term pH of a solution.
[2]
(b) Calculate the pH value of
(i) 0.002M hydrochloric acid.
[2]
(ii)
0.002M
sodium hydroxide solution.
(Kw = [H+] [OH-] =
1.0 x 10-14 mol2 dm-6)
[2]
(c)
Without
doing further calculations, sketch a graph of the approximate pH changes which
occur when excess 0.002M sodium hydroxide solution is added gradually to;
(i) 25 cm3 of 0.002M hydrochloric acid [2]
(ii)
25
cm3 of 0.002M ethanoic acid
[2]
14 ¾
12 ¾
10 ¾
8 ¾
pH
6 ¾
4 ¾
2 ¾
0 | | | | | | | |
5 10 15 20 25 30 35 40
volume
of NaOH (aq) / cm3
(d)
Explain
why phenolphthalein is a suitable indicator for the titration of sodium
hydroxide with ethanoic acid.
[2]
(e)
0.400
mole of ethanoic acid is dissolved in water to give a final volume of 2.00 dm3. Assuming that the extent of ionisation is
small, calculate the pH of the solution, given that the dissociation constant
of the acid is 1.8 x 10-5 mol2 dm-3 at 298K.
[3]
(f)
0.200
mole of sodium ethanoate is added to the above solution. Assuming that any volume change is
negligible, calculate the pH of the solution.
[4]
2.
The
following graph shows the changes in pH as aqueous ammonia is added to 25.0 cm3
of dilute hydrochloric acid in a titration.
(a)
Write
an equation for the reaction between aqueous ammonia and dilute hydrochloric
acid.
[2]
(b)
Use
the graph to deduce
(i)
the
pH, and hence the concentration of the hydrochloric acid at the start of the titration.
[3]
(ii)
the
volume of ammonia added at the end point of the titration.
[1]
(iii)
the
concentration of the aqueous ammonia used in the titration.
[2]
(c)
What
features of the graph enable you to deduce that aqueous ammonia is a weak base?
[2]
(d)
On
what basis would you choose an indicator for use in the titration?
[2]
(e)
Name
a suitable indicator for this titration. Explain your choice.
[2]
TOTAL
50 MARKS
