Chem 204
Spring 2014
Name___________________________________
TA/Section___________________________
PRACTICE EXAM II
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This Examination is worth 100 points.
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There are 9 pages (counting this one) and a periodic table.
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No books, notes, cellphones, anything with earbuds/earphones, or laptops; you may only
have out pencils or pens and a scientific calculator.
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Write directly on the exam.
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You have two hours to complete this exam. You may leave early if you are done early.
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When you turn your exam in, you must show your university ID card to the proctor/TA.
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Constants and conversions that you might need are given on the next page.
Good luck!
1 Constants and Conversions
NA = Avogadro's number = 6.022 x 1023 mol-1
c = 3.00 x 108 m s-1
h = 6.626 x 10-34 J s
charge on one electron = e = 1.602 x 10-19 C
ε0 = 8.854 x 10-12 C2J-1m-1
me = mass of electron = 9.109 x 10-31 kg
1 atomic mass unit = 1.6605 x 10-27 kg
k = kB = Boltzmann’s constant = 1.3805 x 10-23 J K-1
1 eV = 1.602 x 10-19 J
R = 0.08206 L atm K-1 mol-1 = 8.314 J K-1 mol-1
K = 273.15 + ˚C
1 cm-1 = 1.987 x 10-23 J
1 J = 1 kg m2 s-2
Hz = hertz = s-1
1000 L = 1 m3
760 torr = 1 atm = 14.7 psi = 1.01325 bar = 1013.25 mb
Formulas
(P + a(n2/V2)) (V - nb) = nRT
ln (P) = -∆Hvap˚/RT + ∆Svap˚/R
ln(P2/P1) = (∆H˚vap/R) [(1/T1) – (1/T2)]
-b ±
x=
ln(K2/K1) = (∆H˚/R) [(1/T1) – (1/T2)]
b2 - 4ac
________________
for ax2 + bx + c = 0
2a
Equilibrium Constants at 25˚C
AgCl:
AgBr:
AgI:
H3PO4:
Ksp = 1.6 x 10-10
Ksp = 7.7 x 10-13
Ksp = 1.5 x 10-16
Ka (first proton) = 7.52 x10-3
HF: Ka = 6.6 x 10-4
H2PO4-:
Ka (first proton) = 6.23 x 10-8
HCl: Ka ~ 107
HPO42-:
Ka = 2.2 x 10-13
HI: Ka ~1010
H2S:
S2- :
Ka = 9.1 x 10-8
Kb = 105
NH3:
Kb = 1.8 x 10-5
CH3COOH:
Ka = 1.8 x 10-5
HNO3: Ka = 24
H2CO3:
Ka = 4.4 x 10-7
HCO3-: Ka = 4.7 x 10-11
2 Kw = 1.0 x 10-14
I. (50 points). Multiple Choice. Circle the letter of the best answer. 5 points each. No
partial credit.
1. Can the pH of an aqueous solution ever be a negative number?
a. Yes.
b. No.
2. For this problem only: circle ALL the statements that are TRUE.
a. The stronger the base, the smaller the pKb.
b. The stronger the base, the larger the pKb.
c. The stronger the base, the smaller the Kb.
d. The stronger the base, the larger the Kb.
e. The stronger the base, the smaller the pKa of its conjugate acid.
f. The stronger the base, the larger the pKa of its conjugate acid.
3. Which of the molecules below neither an acid nor a base? Circle the whole molecule.
O
H OH
C
H
OH
O
HO
OH
HO
H
OH
H
H
O
H2N
H2N
CH
C
OH
N
N
CH2
C
O
N
H
N
OH
3 4. In nonaqueous solvents, it is possible to react HF to create H2F+. What does this mean?
a. HF can like an even stronger acid in nonaqueous solvents.
b. HF can act like a base in nonaqueous solvents.
c. HF is really unstable.
5. The pKb of water at 298 K is…
a. 1
b. 7
c. 14
d. not defined
e. none of the above
6. During a pH titration, you know you are at the equivalence point when…
a. stuff starts to precipitate out of solution.
b. the pH changes dramatically with only a small addition of your titrant solution.
c. the pKa of the weak acid component is equal to the pH of the solution.
d. the pH is 7.
e. More than one of the above.
f. None of the above.
7. Estimate the pH of a 0.0100 M aqueous solution of NH4Cl.
a. 7
b. less than 7
c. more than 7
8. Estimate the pH of a 0.0100 M aqueous solution of NaNO3.
a. 7
b. less than 7
c. more than 7
9. If a weak carboxylic acid is strongly hydrogen-bonded to another weak carboxylic acid, how
does that affect the ability of the acid to be deprotonated?
a. It does not affect anything.
b. The hydrogen-bonded acid is easier to deprotonate compared to the free acid.
c. The hydrogen-bonded acid is harder to deprotonate compared to the free acid.
10. Which of the statements below is NOT an assumption of the Langmuir adsorption
isotherm?
a. Molecules interact with each other when bound to a surface.
b. Molecules form, at most, a monolayer on the surface.
c. The surface is homogeneous, from the molecules’ point of view.
d. The enthalpy of adsorption is identical, no matter which surface site is occupied.
e. All of these are assumptions of the Langmuir adsorption isotherm.
4 For the remainder of the exam: you must show clear work in order to earn any partial
credit.
II. (12 points). For each molecule or ion, write “acid” “base” “both” or “neither” on the line near
the molecule’s name, to indicate how you predict that the molecule would act, in the BronstedLowry sense, in room temperature aqueous solution.
HCO3-, the bicarbonate ion: _______________________
H
N
Prozac:_____________________________ O
CF3
O
pyruvate:_________________________________________ OH
O
H2N
COOH
O2N
NO2
PABA (formerly in sunscreen):_______________________ TNT, trinitrotoluene:_______________________________ NO2
N-­‐methylpyridinium:_____________________________________ N
5 III. (10 points). Rainwater is acidic because CO2(g) dissolves in the water, creating carbonic
acid, H2CO3. If the rainwater is too acidic, it can do things like dissolve limestone and seashells
(made principally of calcium carbonate, CaCO3). Calculate the concentrations of carbonic acid,
bicarbonate ion (HCO3-) and carbonate ion (CO32-) that is in a raindrop that has a pH of 5.60,
assuming the sum of all carbonate-containing species is 1.0 x 10-5 M in the raindrop.
6 IV. (5 points). Prove that KaKb = Kw.
V. (5 points).
Derive the van’t Hoff equation.
7 VI. (10 points). Baking soda (sodium bicarbonate, NaHCO3) reacts with acids in food to form
carbonic acid (H2CO3), which in turn decomposes to water and carbon dioxide gas. In a cake
batter, the CO2(g) forms bubbles, causing the cake to rise and become nice and fluffy.
(a) A rule of thumb in baking is that ½ teaspoon of baking soda is neutralized by one cup of sour
milk. The acid component in sour milk is lactic acid, CH3CH(OH)COOH. Write an equation for
this neutralization reaction.
(b) The density of baking soda is 2.16 g/cm3. Calculate the concentration of lactic acid in one
cup of sour milk (assuming our rule of thumb applies), in units of mol/L. One cup = 236.6 mL =
236.6 cm3 = 48 teaspoons.
(c) If ½ teaspoon of baking soda is indeed completely neutralized by lactic acid, calculate the
volume of carbon dioxide gas that would be produced at 1 atm pressure, in an oven set to 350
˚F (=177 ˚C).
8 VII. (8 points).
Without doing any calculations, predict how the pH of an aqueous solution of sodium
trifluoroacetate, CF3COO-Na+, would compare to the pH of an aqueous solution of potassium
acetate, CH3COO-K+, at the same concentration: should the pH’s be the same? If not, which
solution will have a higher pH? Explain your reasoning.
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