In-class Problems
• (1) The end point of a titration is defined
as
(a) the equivalence point of the titration.
(b) the actual measured volume of titrant
required to complete a titration.
(c) the volume associated with actual
stoichiometric quantity of titrant required to
complete a
titration.
Lecture 11
2
In-class Problems
In-class Problems
• (2) Which statement concerning titrations
is incorrect?
(a) The titration error can be accounted for
by running a blank titration.
(b) The titrant is a primary standard after
standardization.
(c) The endpoint and equivalence point
are not the same.
• (3) Which of the following pairs of
compounds and ions could be used to
form a buffer solution?
(a) 0.10 M HCl and 0.10 M NaCl
(b) 0.10 M NH4Cl and 0.10 M NH3
(c) 0.10 M HNO2 and 1.0 x 10-4 M NaNO2
3
In-class Problems
4
In-class Problems
• (4) An indicator may be selected for an acidbase application based on the pH range where
the indicator changes color. A chemist wanted
an indicator to change color between pH 5.8 and
6.1. The following compounds were available for
selection: Ina Ka = 1.5 X 10-3, Inb Ka = 1.3 X
10-6, Inc Ka = 1.8 X 10-8. Which would be the
proper indicator to select?
(a) Ina
(b) Inb
(c) Inc
5
• (5) Given the hypothetical acid H4A2+, what
species determines the isoionic pH?
(a) H4A2+
(b) A2(c) H2A
6
1
In-class Problems
In-class Problems
• (6) Given the hypothetical acid H4A2+, the
concentration of which species are equal
at the isoelectric pH?
(a) [H4A2+] = [H3A+].
(b) [H3A+] = [HA-].
(c) [H2A] = [HA-].
(7) Given the hypothetical acid H2A (pKa1 =
3.44 and pKa2 = 8.45), what is the principle
species in solution at pH = 3.50?
(a) H3A+
(b) HA(c) H2A
7
In-class Problems
8
In-class Problems
• (9) Given the transition ranges for the pH
indicators below, which indicator is most
appropriate for the titration of a weak acid with a
strong base?
Cresol purple: pH transition = 1.2-2.8
Bromocresol purple: pH transition = 5.2-6.8
Cresol red: pH transition = 7.2-8.8
(a) Cresol purple
(b) Bromocresol purple
(c) Cresol red
• (8) Sulfurous acid (H2SO3) has
pKa1=1.91 and pKa2=7.18. At what pH
does [HSO3-]=[SO32-]?
(a) 1.91
(b) 4.54
(c) 7.18
9
In-class Problems
10
In-class Problems
• (10) The reaction of EDTA with a trivalent metal
ion at a pH of 4.0 may be written as:
• (11) When is a direct EDTA titration not useful?
(a) When the metal precipitates in the absence
of EDTA.
(b) When the metal reacts too quickly with
EDTA.
(c) When the metal does not block the indicator.
(a) M3+ + Y4- <===> MY-.
(b) M3+ + H2Y2- <==> MY- + 2 H+.
(c) M3+ + H6Y2+<==> MY- + 6 H+.
11
12
2
In-class Problems
In-class Problems
• (12) Calculate the pCo2+ after 13.00 mL
0.03846 M EDTA in the titration of 25.00
mL of 0.020 M Co2+ (Kf=2.04×1016) at pH
6.00. Calculate the pCo2+.
• (13) KMnO4 is used as the titrant in a
potentiometric titration of tin(II) to tin(IV), using a
S.C.E. reference electrode. After the
equivalence point, you would calculate the cell
voltage using the potentials of
(a) the MnO4-/Mn2+ couple and the S.C.E.
(b) the Sn4+/Sn2+ couple and the S.C.E.
(c) the MnO4-/Mn2+ couple and the Sn4+/Sn2+
couple.
13
14
In-class Problems
• (14) Write the two half-reaction of the cell:
Pt(s)|Fe3+(0.001 M), Fe2+(0.100 M)||I(0.001 M), H+ (0.001 M), IO3- (0.1 M)|Pt(s).
The Eo for Fe3+/Fe2+=0.771 V and Eo for
IO3-/I2=1.19 V.
15
3