CHEMISTRY 100 CHRISTMAS EXAM - 2010
Name:
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ID#:
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Prof.:
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PLEASE READ THIS BEFORE YOU BEGIN:
This exam comprises 12 pages (including this page) and a periodic
table. Please make sure you have a
complete exam and that you have filled in the information above. All questions are to be answered on the
question sheets either by circling the appropriate answer on the
multiple-choice questions or by writing in your solutions for the short and
long answer questions. You may use pen
or pencil. Rough calculations can be
performed on the back of the pages. Proper
calculators are permitted. You have 2
hours. Some useful constants and
equations are given below.
Constants
R = 0.08206 L-atm/mol-K OR 8.314 J/mol-K
Avogadro’s number (N) = 6.022142 x 1023 mol-1
Atomic mass unit (amu) = 1.66059 x 10-24 g
STP for a gas: 1 atm and 0°C
101325 Pa = 1 atm
Formulas
Quadratic formula: - b ± (b2 - 4ac)1/2
2a
PV = nRT
u = [(3RT)/(M)]½
pH = pKa + log([base]/[acid])
Kp = Kc(RT)Dn
(I) Multiple Choice (30 marks, 2 marks each)
Please clearly
circle the letter of your answer. If two
or more answers are circled the question will be marked wrong regardless of
whether the correct answer is circled.
1. In which species does sulfur have the highest oxidation number?
a) S8
(elemental form of sulfur) b) H2S c)
SO2 d) H2SO3 e)
K2SO4
2. A gas originally at 27 °C and 1.00 atm pressure in a 3.9 L flask is cooled at constant pressure until the temperature is 11 °C. The new volume of the gas is ________ L.
a) 0.27 b) 3.7 c) 3.9 d) 4.1 e) 0.24
3. A tank containing both HF and HBr gases developed a leak. The ratio of the rate of effusion of HF to the rate of effusion of HBr is ________.
a) 4.04 b) 0.247 c) 2.01 d) 0.497 e) 16.3
4. What is the pH of an aqueous solution at 25 °C that contains 3.98 x 10-9 M hydroxide ion?
a) 8.40 b) 5.60 c) 9.00 d) 3.98 e) 7.00
5. The Ka for HF is 7.0 x 10-4. The Kb for the fluoride ion is ________.
a) 2.0 x 10-8 b) 1.4 x 10-11 c) 7.0 x 10-18 d) 7.0 x 10-4 e) 1.4 x 103
6. The hydride ion, H-, is a stronger base than the hydroxide ion, OH-. The product(s) of the reaction of hydride ion with water is (are) ________.
a) H3O+ (aq) b) H2O2 (aq) c) OH- (aq) + 2H+ (aq) d) OH- (aq) + H2 (g)
e) no reaction occurs
7. There are ____ moles of carbon atoms in 4 mol of C2H6SO
a) 2 b) 6 c) 8 d) 4 e) 3
8. CH3OH (g) D CO (g) + 2H2 (g) DH = + 90.7 kJ/mol
In order to maximize the equilibrium yield of CO, you could
a) decrease T b) increase the volume of the vessel c) use a catalyst d) increase P
e) add a noble gas such as He
9. For an atom of the element 146C, the number 14 is known as the element’s
a) atomic number b) isotope label c) neutron count d) mass number e) period
10. Which of the following is a metal?
a) hydrogen b)
selenium c) radon
d) antimony e)
tungsten
11. Which of the following statements is true?
a) 1/12th
the mass of 1 atom of 12C is 1 amu
b) there are 6 neutrons in a 11C nuclide
c) 1g 12C contains 6.02 x 1023 atoms
d) ammonium is a polyatomic anion
e) plumbous ion is another name for Cu(II)
12. Which of the following elements is not written in its standard state?
a) P4(s) b) Hg(s) c) Fe(s) d) Br2(l) e) I2(s)
13. A substance (for which pKa = 2.82) is titrated to its equivalence point with sodium hydroxide. What is the pH of the resulting solution?
a) neutral b) weakly basic c) weakly acidic d) strongly basic
e) not enough data to tell
14. Which one of the following pairs could be
mixed together to form a buffer solution?
a) CaCl2, NH3 b) NaC2H3O2,
H2C2O4
c) RbOH, HBr d)
NaCN, NH4Cl
e) H3PO4,
KH2PO4
15. Given
C3H8(g) + 5O2(g) à 3CO2(g) + 4H2O(l) DHo = -2219.9 kJ
2C(graphite) + 2O2(g) à 2CO2(g) DHo = -787 kJ
H2O(l) à H2(g) + ½O2(g) DHo = 285.8 kJ
what is the enthalpy change for the following reaction?
3C(graphite) + 4H2(g) à C3H8(g)
a) -3293 kJ b) +2721 kJ c) -104 kJ d) +754 kJ e) -538 kJ
3/2 (-778) + 4(-285.5) + 2219.9= -104
(II) Short Answer Questions (20 marks, each
worth 4 marks). Answer in the space
provided.
1. Gallium consists of two naturally occurring isotopes with masses of 68.926 and 70.925 amu. The average atomic mass of gallium is 69.72. amu. Calculate the abundance of each isotope
69.72 = 68.926x + 70.925 (1-x); x=0.6028; 1-x = 0.3977
60.28% and 39.77%
2. Car batteries contain a solution of sulfuric acid which is commonly named as ‘battery acid’. Calculate the number of grams of sulfuric acid in 2.50 L of battery acid if the solution has a density of 1.28g/mL and is 38.1% sulfuric acid by mass.
1.28
g/mL x 2500 mL x 0.381 = 1219 g
3. What is the ratio of H2CO3
to HCO3- in blood of pH = 7.40? (Ka1 = 4.3 x 10-7)
Ka = [HCO3-] [H+] / [H2CO3]; [H2CO3]/[HCO3-] = [H+]/Ka = 3.98 x 10-8/4.3 x 10-7 = 0.0925
[H+] = 10-7.40 = 3.98 x 10-8
4. To minimize the rate of evaporation of the tungsten filament, 1.4 x 10-5 mol of argon is placed in a 550 cm3 light-bulb. What is the pressure of the argon in the light-bulb at 25 °C?
P= nRT/V = (1.4 x 10-5mol x 298 K x 0.08206 L atm/mol K)/0.550L = 6.22 x 10-4 atm
5. At 900K, the equilibrium constant (Kp) for the following reaction is 0.345.
2SO2 (g) + O2 (g) D 2SO3 (g)
At equilibrium, the partial pressure of SO2 is 35.0 atm and that of O2 is 15.9 atm. Calculate the total pressure of the gas mixture at equilibrium.
Kp
= (PSO3)2/(PSO2)2xPO2 =
0.345
PSO3 = (PSO2)2 x 0.345 x PO2
)1/2 = ( 0.345 x 1225x 15.9)1/2 = (6719.7)1/2
= 82.0 atm
Ptotal = 82.0 + 35.0 + 15.9 = 132.9 atm
(III) Longer Questions (50 marks, each worth
10). Answer in the space provided.
1. a) How many grams of potassium benzoate (KC7H5O2) should be added to 500.0 mL of 0.150 M benzoic acid to form a buffer solution with a pH = 3.900? (Assume that no volume change occurs when the KC7H5O2 is added). Ka = 6.30 x 10-5.
pH = pKa + log [base] –log [acid] pKa = -log 6.30 x 10-5 = 4.2
3.9 = 4.2 - log 0.15+log base; log base = -4.2 +3.9 -0.824 = -1.124
[base] = 10-1.124 =0.0752M
mol = 0.0752 mol/L x 0.5 L = 0.0376 mol
M KC7H5O2 = 160.2 g/mol: g KC7H5O2 = 160.2 g/mol x 0.0376 mol =6.02 g
b) What would the pH of this buffer be after the addition of 0.00400 moles of NaOH?
before acid 0.150mol/L x 0.5L =0.075mol base 0.0376 mol
after 0.071mol 0.0416
pH = 4.2 + log (0.0416/0.071) =
3.97
2. The acid-dissociation constant, Ka,
for hydrocyanic acid (HCN) is 4.9 x 10-10.
a) What is the pH of an aqueous solution made by dissolving 1.25 g of hydrocyanic acid in 150.0 mL of water?
HCN D H+ + CN- Ka = [H+][CN-] / [HCN]
1.25g / 27.03 g mol-1 = 4.62 x 10-2 mol
4.62 x 10-2 mol / 0.150 L = 0.308 M
Using the reaction table
HCN D H+ + CN-
I 0.308 0 0
C -x +x +x
E 0.308 – x x x
Assuming 0.37 – x » 0.308 then:
x2 / 0.308 = 4.9 x 10-10 x = 1.23 x 10-5 = [H+] pH = -log[H+] = 4.91
b) What is the % ionization of hydrocyanic acid at this concentration?
% ionization = (1.35 x 10-5 / 0.308 ) x 100 = 0.0044 %
3. Combustion of a 1.031-g sample of a compound containing only C, H, and O produced 2.265g of CO2 and 1.236 g of H2O. What is the empirical formula of the compound?
mol CO2 = 2.256/44.01 = 0.05126 = mol C
gC = 0.05126 x 12.01 = 0.6156 g C
mol H2O
= 1.236/18.016 = 0.06861 mol
mol H =
2x0.06861mol = 0.1372 molH
gH =0.1372
x 1.008 =0.1383 gH
gO = 1.031g
– 0.1383g – 0.6156g = 0.2771g O
mol O =
0.2771/16 =0.0173 mol
O: 0.0173/0.0173=1; H: 0.1372/0.0173=8; C: 0.05126/0.0173 = 3
EF: C3H8O
4. Calculate the solubility of LaF3 (Ksp = 2 x 10-19) in grams per liter in:
a) pure water
La F3 (s) D La 3+ (aq) + 3F- (aq) Ksp = [La3+][F-]3 = 2x10-19 = X x [3X]3 = 27X4
X= ( 2 x 10-19/27)1/4 = 9.28 x 10-6 M; MM LaF3 = 196 g/mol; g/L: 9.28 x 10-6 mol/L x 196 g/mol=
1.82 x 10-3 g/L
b) 0.010 M KF solution
La F3 (s) D La 3+ (aq) + 3F- (aq) Ksp = [La3+][F-]3 = 2x10-19
- 0.01M
x 3x
x 0.01 +3x
2x10-19 = X x (0.01 +3X)3 ≈ X x (0.01)3 = 1.0 x 10-6
X= (2x10-19)/ 1.0 x 10-6 = 2 x 10-13 M
196 g/mol x 2 x 10-13 mol/L = 3.92 x 10-11 g/L
5. Complete and balance the following equation in basic solution and identify the oxidizing and reducing agents.
MnO4- (aq) + Br- (aq) ¾¾® MnO2 (s) + BrO3- (aq)
Oxidizing agent _______________
Reducing agent _______________
MnO4- (aq) + Br- (aq) ¾¾® MnO2 (s) + BrO3- (aq)
2[MnO4- (aq) + 2H2O (l) + 3e- ¾® MnO2 (s) + 4OH-]
Br- (aq) + 6OH- (aq) ¾® BrO3- (aq) + 3H2O (l) + 6e-
-------------------------------------------------------------------------------
2MnO4- (aq) + Br- (aq) + H2O (l) ¾® 2MnO2 (s) + BrO3- (aq) + 2OH- (aq)
Oxidizing agent: MnO4-
Reducing agent: Br-