1) Ch.17 & 20 equilibrium III & electrochemistry (Partners: Ralph, Karen &
Gloria)
1. What is the pH of a 45mL aqueous solution containing 25 mM sodium acetate and
35mM acetic acid after the addition of __.
a. 10 mL of water
While addition of water changes [ ], it doesn’t change ratio of [ ] in H-H
solution.
pH = pKa + log([A-]/[HA]) = -log(1.8x10^-5) + log(25mM/35mM)
= 4.745 – 0.146 = 4.60
b. 10 mL of 50. mM HCl
i)
nHCl = [ ]V = 50mmol/10^3mL(10mL) = 0.50 mmol
nAc- = [ ]V = 25mmol/10^mL(45mL) = 1.125 mmol
nHA = [ ]V = 35mmol/10^3mL(45mL) = 1.575 mmol
ii)
H+
+
Ac
HAc
I(mmol)
0.5
1.125
1.575
C(mmol)
-0.5
-0.5
+0.5
E(mmol)
0
0.625
2.075
iii)
pH = pKa + log([A]/[HA])
= 4.745 + log(0.625/2.075)
= 4.22
c. 10 mL of 50 mM KOH
i)
nOH= [ ]V= 50mmol/10^3mL(10mL) = 0.5 mmol
ii)
OH+
HAc

AcI(mmol)
0.5
1.575
1.125
C(mmol)
-0.5
-0.5
+0.5
E(mmol)
0
1.075
1.625
iii)
pH = pKa + log([Ac-]/[HAc])
= 4.745 + log(1.625/1.075)
= 4.92
2. What is the minimum volume of a 0.1 mM NaCl solution to be added to a
25.0x10^-6 M 50.0mL solution of silver nitrate to produce a precipitate? (free
points)
Ppt @ Q > Ksp
[Cl-][Ag+] > Ksp
[Cl-]VCl-/(Vtotal) x [Ag+]VAg/(Vtotal) > Ksp
0.1x10^-3mol/10^3mL(VCl-)/(50mL + VCl-) x (25x10^6mol/10^3mL)(50mL)/(50mL + VCl-) > Ksp
10^-7x(1.25x10^-6)/(50+x)2 > Ksp
1.25x10^-13x/(2500+100x+x2) > 1.8 x 10^-10
1.25*10^-13x > 4.5x10^-7 + 1.8x10^-8x + 1.8*10^-10 (x2 )
1.8*10^-10x2 + 1.8*10^-8 (x) + 4.5x10^7
X < -50mL ?
3. What is the relative solubility of AgCl in water versus the solubility of AgCl in 0.1
mM NaCl? Rationalize/justify your answer without doing any calculations.
Solubility in H2O > solubility in NaCl.
4. How much time(in seconds) would it take to deposit 125 mg of zinc onto a carbon
electrode with a current of 275mA?
Zn+2 + 2e-  Zn
i)
125mgZn(1g/10^3mg)(1molZn/63.55gZn)(2molee-/1molZn)(96,500
Coulombs/1mole e-) = 380 coulombs
ii)
I = #coulombs/time
275mA (A/10^3mA) = 380 coulombs/time
Time= 1.38x10^3 sec
5. Sketch the voltaic cell for the redox reaction: Zn + Cu(NO3)2(aq)  Cu +
Zn(NO3)2(aq). Clearly label (i) the identity of the electrode(s), (ii) electrolyte(s) in
the cell, (iii) the half-reactions at the electrode, (iv) direction of the current, and (v)
cathode versus anode in the electrochemical cell.
Note: current flows in the opposite direction of e- flow, so current flows to the
left.
6. Is the following redox reaction spontaneous(under standard conditions): ___;
justify/rationalize your answer.
a. Zn + HCl(aq)  [hint: it’s a single replacement reaction]
i)
Zn  Zn+2 + 2e- E° = +0.763V
2e- + 2H+  H2
E° = 0
E°cell = 0.763 V
ii)
ΔG° = -nFE°
E° > 0  ΔG ° < 0  Rx spontaneous
b. Cu + HCl(aq)  Cu+2
i)
Cu Cu+2 + 2e2e- + 2H+  H2
ii)
E° = -0.337
E° = 0
E°cell = -0.337 V
ΔG° = -nFE°
E° < 0  ΔG°> 0  Rx not spontaneous
************************************************************
Gloria Lau and Karen Wong
Equilibrium III & Electrochemistry
1. What is the pH of a 45mL aqueous solution containing 25 mM sodium and 35 mM acetic
acid after the addition of __.[20 points]
a. 10mL of water
addition of water change [ ]; doesn’t change rxn of [ ] in H-H equ
pH = pKe + log
= -log (1.8E-5) + log
b. 10mL of 50.0 mM HCl
= 4.60
i.
nHCl = [ ]V =
10mL = 0.50 mM
nAc = [ ]V =
45mL = 1.123 mM
nHA = [ ]V =
H+ + Ac-
ii.
45mL = 1.575 mM
HAc
I
0.5
1.125
1.575
C
-0.5
-0.5
+0.5
E
0
0.625
2.075
pH = pKe + log
iii.
= 4.22
c. 10mL of 50mM KOH
nHCl = [ ]V =
i.
OH- + HAc
ii.
10mL = 0.50mM
Ac-
I
0.5
1.575
1.125
C
-0.5
-0.5
+0.5
E
0
1.075
1.625
pH = pKe + log
iii.
= 4.92
2. What is the minimum volume of a 0.1mM NaCl solution to be added to a 25.0
solution of silver nitrate to produce a precipitate? [10 points]
ppt @ Q > Ksp
> Ksp
50.0mL
[Cl-][Ag+] > Ksp
> 1.8E-10
> Ksp
x > -90mL
3. What is the relative solubility of AgCl in water versus the solubility of AgCl in 0.1 mM
NaCl? ratioalize/ justify your answer without doing any calculations. [10 points]
solubility in H2O > solublity in NaCl
4. How much time (in sec) would it take to deposit 125mg of Zinc onto a carbon electrode with
a current of 275 mA? [10 points]
Zn2+ + 2e-
Zn
125mg Zn
i.
I=
ii.
= 380 cal
= 275mA
=
t = 1.38E3 sec
5. Sketch the voltaic cell for the redox reaction: Zn + Cu(NO3)2
Cu + Zn(NO3)2. Clearly label
(i) the identity of the electrode, (ii) electrolyte(s) in the cell, (iii) the half-reaction at the
electrode, (iv) direction of the current, and (v) cathode versus anode in the electrochemical
cell. [10 points]
Zn2+ + 2e-
Zn
Cu2+ + 2e-
anode
Cu
cathode
6. Is the following redox reaction spontaneous (under standard condition): __; justify/
rationalize your answer. [10 points]
a. Zn + HCl(aq)
i.
[hint: it’s a single replacement reaction]
Zn2+ +2e-
Zn
2e- + 2H+
Eo = +0.763
Eo = 0
H2
Eotot = 0.763
o
ii.
= -nFEo
Eo. > 0
<0
Rx spontaneous
b. Cu + HCl(aq)
i.
Cu
Cu2+ + 2e-
2e- + 2H+
Eo = -0.337
Eo = 0
H2
Eotot = -0.337
ii.
= -nFEo
Eo = < 0
>0
Rx not spontaneous
7. Identify the species (i.e. “atom”) that is oxidized or reduced in the following redox reaction
and its oxidation number. [10 points]
a. Decomposition of hydrogen peroxide
b. Cu + K2Cr2O7(aq) + H+(aq)
Cr3+(aq) + H2O + Cu2+(aq)
8. Old topic: describe and rationalize how to determine the Ka of benzoic acid based in __. [10
points]
a. the pH of the benzoic acid solution
i.
ii.
HBz
H+ + BzI
HBz
0
0
C
-x
-x
+x
E
HBz - x
-x
x
Ka =
b. Benzoic acid titration curve using a strong base, NaOHVb = Vb
pH = pKe + log
pH = pKa
[H+] = Ka