CHEMISTRY 100 CHRISTMAS EXAM - 2010

Name: _______________________

ID#: _________________________

Prof.: ________________________

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 = pK_a + log([base]/[acid])

K_p = K_c(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 K_a for HF is 7.0 x 10^-4. The K_b 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) 1/12^th 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 pK_a = 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) DH^o = -2219.9 kJ

2C(graphite) + 2O₂(g) à 2CO₂(g) DH^o = -787 kJ

H₂O(l) à H₂(g) + ½O₂(g) DH^o = 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? (K_a1 = 4.3 x 10^-7)

K_a = [HCO₃^-] [H⁺] / [H₂CO₃]; [H₂CO₃]/[HCO₃^-] = [H⁺]/K_a = 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 (K_p) 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.

K_p = (PSO₃)²/(PSO₂)²xPO₂ = 0.345

P_SO3 = (P_SO2)² x 0.345 x P_O2 )^1/2 = ( 0.345 x 1225x 15.9)^1/2 = (6719.7)^1/2 = 82.0 atm

P^total = 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). K_a = 6.30 x 10^-5.

pH = pK_a + log [base] –log [acid] pK_a = -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, K_a, 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^- K_a = [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 (K_sp = 2 x 10^-19) in grams per liter in:

a) pure water

La F₃ (s) D La ³⁺ (aq) + 3F^- (aq) K_sp = [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) K_sp = [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^-