Recitation Ch. 17
Name_______________________________
(A)CO32– (C) NH4+ (E) Al3+
(B)Cr2O72– (D)Ba2+
Assume that you have an “unknown” consisting of an aqueous solution of a salt that contains one of the
ions listed above. Which ion must be absent on the basis of each of the following observations of the
“unknown”?
1.
The solution is colorless
2.
The solution gives no apparent reaction with dilute hydrochloric acid.
3.
No odor can be detected when a sample of the solution is added drop by drop to a warm
solution of sodium hydroxide.
4.
No precipitate is formed when a dilute solution of H2SO4 is added to a sample of the solution.
5.) Acid
Acid Dissociation Constant, Ka
H3PO4
710–3
H2PO4–
810–8
2–
HPO4
510–13
On the basis of the information above, a buffer with a pH = 9 can best be made by using
(A) pure NaH2PO4
(D) H2PO4– + HPO42–
(B) H3PO4 + H2PO4–
(E) HPO42– + PO43–
–
2–
(C) H2PO4 + PO4
a. a solution with a pH less than 7 that is not a buffer solution
b. a buffer solution with a pH between 4 and 7
c. a buffer solution with a pH between 7 and 10
d. a solution with a pH greater than 7 that is not a buffer solution
e. a solution with a pH of 7
(Ionization constants:
CH3COOH = 1.810–5;
NH3 = 1.810–5;
H2CO3:
K1 = 410–7;
K2 = 410–11;)
6.
A solution prepared to be initially 1 M in NaCl and 1 M in HCl.
7.
A solution prepared to be initially 1 M in Na2CO3 and 1 M in CH3COONa
8.
A solution prepared to be initially 0.5 M in CH3COOH and 1 M in CH3COONa
9.) In the titration of a weak acid of unknown concentration with a standard solution of a strong
base, a pH meter was used to follow the progress of the titration. Which of the following is
true for this experiment?
a. The pH is 7 at the equivalence point.
b. The pH at the equivalence point depends on the indicator used.
c. The graph of pH versus volume of base added rises gradually at first and then much
more rapidly.
d. The graph of pH versus volume of base added shows no sharp rise.
e. The [H+] at the equivalence point equals the ionization constant of the acid.
10.) When phenolphthalein is used as the indicator in a titration of an HCl solution with a
solution of NaOH, the indicator undergoes a color change from clear to red at the end point
of the titration. This color change occurs abruptly because
a. phenolphthalein is a very strong acid that is capable of rapid dissociation
b. the solution being titrated undergoes a large pH change near the end point of the titration
c. phenolphthalein undergoes an irreversible reaction in basic solution
d. OH– acts as a catalyst for the decomposition of phenolphthalein
e. phenolphthalein is involved in the rate–determining step of the reaction between H3O+
and OH–
11.) The solubility of CuI is 210–6 molar. What is the solubility product constant, Ksp, for CuI?
a. 1.410–3
c. 410–12
e. 810–18
–6
–12
b. 210
d. 210
12.) Commercial vinegar was titrated with NaOH solution to determine the content of acetic
acid, HC2H3O2. For 20.0 milliliters of the vinegar, 26.7 milliliters of 0.600–molar NaOH
solution was required. What was the concentration of acetic acid in the vinegar if no other
acid was present?
(A)
1.60 M
(B)
0.800 M
(C)
0.600 M
(D)
0.450 M
(E)
0.200 M
13.) Barium sulfate is LEAST soluble in a 0.01–molar solution of which of the following?
(A)Al2(SO4)3 (B) (NH4)2SO4
(C)Na2SO4 (D) NH3
(E) BaCl2
14.) Correct
procedures for a titration include which of the following?
I. Draining a pipet by touching the tip to the side of the container used for the titration
II. Rinsing the buret with distilled water just before filling it with the liquid to be titrated
III. Swirling the solution frequently during the titration
(A) I only
(B)II only
(C)I and III only
(D)II and III only
(E) I, II, and III
Written Response:
1.) A sample of 40.0 milliliters of a 0.100 molar HC2H3O2 solution is titrated with a 0.150 molar NaOH
solution. Ka for acetic acid = 1.8x10-5
(a) What volume of NaOH is used in the titration in order to reach the equivalence point?
(b) What is the molar concentration of C2H3O2- at the equivalence point?
(c) What is the pH of the solution at the equivalence point?
2.) (a) Specify the properties of a buffer solution. Describe the components and the composition
of effective buffer solutions.
(b) An employer is interviewing four applicants for a job as a laboratory technician and asks
each how to prepare a buffer solution with a pH close to 9.
Archie A. says he would mix acetic acid and sodium acetate solutions.
Beula B. says she would mix NH4Cl and HCl solutions.
Carla C. says she would mix NH4Cl and NH3 solutions.
Dexter D. says he would mix NH3 and NaOH solutions.
Which of these applicants has given an appropriate procedure? Explain your answer,
referring to your discussion in part (a). Explain what is wrong with the erroneous
procedures.
(No calculations are necessary, but the following acidity constants may be helpful: acetic
acid, Ka= 1.8x10-5; NH4+, Ka = 5.6x10-10)
3.) NH3 + H2O <=> NH4+ + OH- Ammonia is a weak base that dissociates in water as shown above. At
25ºC, the base dissociation constant, Kb, for NH3 is 1.8x10-5.
(a) Determine the hydroxide ion concentration and the percentage dissociation of a 0.150 molar
solution of ammonia at 25ºC.
(b) Determine the pH of a solution prepared by adding 0.0500 mole of solid ammonium
chloride to 100. millilitres of a 0.150 molar solution of ammonia.
(c) If 0.0800 mole of solid magnesium chloride, MgCl2, is dissolved in the solution prepared in
part (b) and the resulting solution is well-stirred, will a precipitate of Mg(OH)2 form? Show
calculations to support your answer. (Assume the volume of the solution is unchanged. The
solubility product constant for Mg(OH)2 is 1.5x10-11.
4.) The equations and constants for the dissociation of three different acids are given below.
HCO3- <=> H+ + CO32-
Ka = 4.2 x 10-7
H2PO4- <=> H+ + HPO42-
Ka = 6.2 x 10-8
HSO4- <=> H+ + SO42-
Ka = 1.3 x 10-2
(a) From the systems above, identify the conjugate pair that is best for preparing a buffer with a
pH of 7.2. Explain your choice.
(b) Explain briefly how you would prepare the buffer solution described in (a) with the
conjugate pair you have chosen.
(c) If the concentrations of both the acid and the conjugate base you have chosen were doubled,
how would the pH be affected? Explain how the capacity of the buffer is affected by this
change in concentrations of acid and base.
(d) Explain briefly how you could prepare the buffer solution in (a) if you had available the
solid salt of the only one member of the conjugate pair and solution of a strong acid and a
strong base.