Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109

 

Chem 109, Dr. Paselk

Final Exam
 Name
Fall 1994

(200 pts)
 Lab

Key

Answers are indicated in red.

Part I (75 pts): Circle the correct answer for each of the following 25 questions (three points each).

1. The formal charge of bromine in BrO3- is:

a. +1 b. -2 c. +2 d. 0 e. none of these (+3)

2. How many grams of NaCl are there in 67.8 ml of a 2.50 M solution?

a. (0.0678)(2.50)/58.44 b. 58.44/(0.0678)(2.50) c. none of these d. (67.8)(2.50)/58.44 e. (0.0678)(2.50)(58.44)

3. The correct formula for Barium phosphate is:

a. BaPO4 b. Ba(HPO4)2 c. BaHPO3 d. Ba3(PO4)2 e. none of these.

4. What is the oxidation number of Cr in Na2Cr2O7?

a. +3 b. +2 c. +4 d. +5 e. none of these (+6)

5. The total electron spin in the ground-state Ru atom is:

a. 0 b. 1/2 c. 2/2 d. 3/2 e. none of these (4/2)

6. Which of the following has the second lowest electronegativity?

a. Fr b. O c. Ra d. Cs e. F

7. The hardest known substances are:

a. metallic solids b. ionic solids c. molecular solids d. covalent solids e. none of these

8. How many s electrons (total) are there in one Ag2+ ion?

a. 8 b. 11 c. 9 d. 13 e. none of these

9. Which of these electronic configurations represents a Group VIB ion?

a. [Xe]6s06d5 b. [Kr]5s04d5 c. [Kr]4d54s2 d. [Kr]4d45s2 e. none of these

10. How many angular nodes are there in a 4f orbital?

a. 1 b. 2 c. 3. d. 4 e. none of these

11. How many electrons are there in 235U2+?

a. 94 b. 90 c. 236 d. 92 e. none of these

12. The bond angles in CO2 are all about:

a 90° b. 109° c. 120° d. 180° e. 60°

13. _______ should be tetrahedral in shape.

a. H2O b. NH4+ c. BH3 d. BeCl2 e. none of these

14 Which of the following has the lowest freezing point?

a. 2M KCl b. 1.5M MgBr2 c. 2.5M sugar d. 2M H2SO4 e. 1M AlCl3

15. The mass, in grams, of one mole of sodium chlorite is:

a. 122.55 b. 106.45 c. 74.45 d. 90.45 e. none of these

16. Which of the following gases would you expect to show the least deviation from ideal behavior?

a. H2 b. CO c. Xe d. F2 e. Rn

17. A molecule of MW = 312 has a simplest formula of C4H8O3. What is its molecular formula?

a. C8H16O6 b. C3H8O3 c. C10H24O9 d. C12H24O10 e. none of these (C12H24O9)

18. The symbol for molality is:

a. M b. m c. % d. F e. none of these

19. Consider the equilibrium reaction: N2 (g) + 3 H2 (g) 2 NH3 (g). If the volume of the container is increased the reaction shifts:

a. to the right. b. to the left. c. up. d. down. e. none of these

20. The pH of a 0.0006 M solution of LiOH is:

a. 2.22 b. 3.22 c. 6.00 d. 4.77 e. none of these (basic)

21. One chlorine ion weighs:

a. (35.45)/(6.02 x 1023) amu b. (35.45)(6.02 x 1023) amu c. 35.45 g d. (35.45)/(6.02 x 1023) g e. none of these

22. The process of gaining electrons in a chemical reaction is called:

a. oxidation b. neutralization c. reduction d. formation e. none of these

23. The critical temperature for a gas is:

a. the temperature at which it ignites in oxygen. b. the same as its condensation temperature. c. the temperature at which it is in equilibrium with its liquid and solid states. d. the highest temperature at which it can be liquified.

24. Which of the following would you expect to be an ionic compound?

a. PCl3 b. OF2 c. CaO2 d. CO e. none of these

25. A Brønsted-Lowry base:

a. donates electrons b. donates protons c. accepts electrons d. accepts protons e. none of these

 

Part II (75 pts): Circle the correct answer for each of the following 15 questions (five points each).

1. The rate of diffusion of helium gas is 6.35 times faster than the rate of diffusion of an unknown molecule. What is the molecular weight of the unknown molecule?

a. 19 b. 25 c. 81 d. 161 e. none of these

ra/rb = (Mb/Ma)1/2; (M)1/2 = (4)1/2(6.35); square both sides, M = 4(40.32) = 161

2. What is the empirical formula of a compound containing 79.99% Mo and 20.01% O?

a. MoO b. MoO2 c. Mo2O3 d. Mo3O4 e. Mo4O5

Assume 100g;

for Mo: (79.99g)/(95.94g/mol) = 0.834/0.834 = 1;

for O: (20.01g)/(16.00g/mol) = 1.251/0.834 = 1.50;

therefore: MoO1.5; so Mo2O3

3. A sample of copper with atomic weight 64.1 consists of an isotopic mixture of 63Cu (mass = 62.5amu) and 65Cu (mass = 64.9 amu). What is the percent of 65Cu in this sample?

a. 90.3 b. 99.3 c. 97.7 d. 77.9 e. none of these (66.7)

x(64.9) + (100–x)(62.5) = 100(64.9)

multiplying and gathering unknown vs. known:

x(64.9 – 62.5) = 100(64.1 – 62.5) = 160

x = 160/2.4 = 66.7

4. The energy of a photon of light with wavelength of 537 nM is:

a. 3.70 x 10-26 Joules b. 3.70 x 10-28 Joules c. 3.70 x 10-19 Joules d. none of these

E = hf = hc/Llambda;

E = (6.626 x 10–34J*s)(2.998 x 108m/s)/(537 x 10-9m) = 3.699 x 10-19J = 3.70 x 10-19J

5. The molecular weight of a gas with a density of 3.67 g/L at 26°C and 1.00 atm is:

a. 82.2 b. 14.95 c. 90.1 d. 7.83 e. none of these.

PV = nRT; n/V = P/RT;

n/V = (1.0atm)/(0.0821L*atm/mol*K)(299.15K) = 4.074 x 10-2mol/L;

density =( 3.67g/L)/(4.074 x 10-2mol/L) = 90.1g/mol

6. What will be the pressure of an ideal gas if 0.210 moles occupies 801 mL at 300.0°C?

a. 6.46 atm b. 1.23 atm c. 3.13 atm d. 12.3 atm e. none of these

PV = nRT; P =nRT/V;

P = (0.210mol)(0.0821L*atm/mol*K)(573.15K)/(0.801L) = 12.3atm

7. Consider the reaction below. If 2.0 moles of oxygen reacts with an excess of C6H16, how many moles of carbon dioxide will be produced?

C6H16 + 10 O2 6 CO2 + 8 H2O

a. 2.4 b. 1.2 c. 6.0 d. 7.0 e. none of these

(2mol O2)(6mol CO2)/(10mol O2) = 1.2mol carbon dioxide

8. If the density of liquid ammonia is 1.43 g/mL, what is the concentration of NH3 in moles per liter?

a. 55.5 M b. 8.64 x 10-5 c. 25.0 d. 86.4 e. none of these (83.95)

(1.43g/mL)(1000mL/L)/(17.03g/mol) = 83.95g/mol = 83.4g/mol

9. How many O atoms are there in 32.0 g of H2SO4?

a. 2.0 x 1023 b. 1.16 x 1023 c. 8.0 x 1023 d. 4.1 x 1023 e. none of these (7.86 x 1023)

{(32.09g H2SO4)/(98.09g H2SO4/mol)}{6.022 x 1023 H2SO4/mol)(4 O/H2SO4)} = 7.86 x 1023

10. What is the percent of sulfur by weight in CuSO4*6H2O?

a. 12.0% b. 13.5% c. 0.12% d. 20.0% e. none of these

(32.0g/S)/({63.56 + 32.00 +4(16.00) + 6(18.00)}/CuSO4*6H2O) = 0.12; 0.12 x 100% = 12.0%

11. Select the correct set of coefficients for the balanced equation:

C3H6O2 N + O2 CO2 + H2O + NO

a. 2,7,6,6,2 b. 1,4,2,4,1 c. 1,4,3,3,1 d. 2,11,6,6,3 e. none of these

12. What is the mass of a small pebble traveling at 8.99 x 10-10 meters per second if it has a wavelegth of 0.081 nm?

a. 8.99 x1012 kg b. 9.10 x103 kg c. 9.10 x10-15 kg d. 8.99 x106 kg e. none of these

lambda = h/mf; m = h/lambdaf;

m = (6.626 x 10–34kg*m2/s)/(0.081 x 10-9m)(8.99 x 10-10m/s) = 9.09 x 10-15kg = 9.09 x 10-12g

13. How many moles of Pb are in 35.4 g of Pb2O3?

a. 9.210 x1022 b. 0.153 c. 7.64 x10-2 d. 0.230 e. none of these

(35.4g)/(2(207.2) + 3(16.00))g/mol = 0.7656mol Pb2O3;

but 2mol Pb/mol Pb2O3 so 2(0.7656mol) = 0.1531mol Pb = 1.53mol Pb

14. What is the heat capacity of a calorimeter that contains 125.0 g of water, if it took 40.0 kJ to raise the temperature of the calorimeter and the water 14.00 °C? (The heat capacity of water is 4.184 Jg-1°C-1)

a. 2.33 x103 J°C-1 b. 2.86 x103 J°C-1 c. 12.0 kJ°C-1 d. 523 J°C-1 e. none of these

q = qwater + qcalorimeter; q = (125.0 g)(14°C)(4.184 Jg-1°C-1) + Ccalorimeter(14°C) = 40.0kJ;

Ccalorimeter(14°C) = 32.678kJ; Ccalorimeter = (32,678J)/(14.00°C) = 2.33 x103 J/°C

15. How much 2.0 M NaOH is needed to make 600 mL of a solution with a pH of 11.00? (Assume the remainder of the solution is water.)

a. 10.0 mL b. 3.00 mL c. .030 mL d. 35.0 mL e. none of these (0.30 mL)

pH = 11.00, so pOH = 3.00, & [OH-] = 1.0 x10–3M;

M1V1 = M2V2;

V2 = V1(M1/M2) = (600mL)(1.0 x10–3M/2.0M) = 0.30mL = 0.3mL

 

Part III (50 pts) Complete the the work requested in the following questions and problems (point values given for each)

(18) 1. Calculate the hydrogen ion concentration and pH in each of the following solutions. Show work for full credit!

a. A solution made up by diluting 7.5 ml of 2.5 M HCl to 350.0 ml.

(7.5 mL)(2.5 M) / 350.0 mL) = 0.054 M; pH = -log [H+] = 1.27

b. The solution resulting from mixing 20.00 ml of 0.25 M H2SO4 and 40.00 mL of 0.125 M HC2H3O2 with 100.00 ml of 0.50 M KOH.

Excess base, so all acid consumed:

HC2H3O2 + H2SO4 + 3 OH- C2H3O2- + SO42- + H2O

moles OH- = (0.1000)(0.50) - (2)(0.02)(0.25) - (0.040)(0.125) = 0.035

[OH-] = 0.035 / 0.160 = 0.219 M; pOH = 0.66

pH = 13.34

c. The solution resulting from adding 50.0 ml of 1.00 M acetic acid and 25.0 g of sodium acetate (C2H2O2-) to enough water to make-up 500.0 ml. (pKa = 4.74)

[HOAc] = (50/500) (1.00 M); [OAc-] = (25.0 g) / 82.03 g/mol) / 0.5000 = 0.610 M

pH = pKa + log [A-] / [HA] = 4.74 + log (0.610 / 0.100)

pH = 5.525

(12) 2. A chemical reaction was found to have an activation energy of 225 kJ mol-1. Calculate the rate for this reaction at 97 °C if it has a value of 1.43 x 10-2 at 25 °C. Show work for full credit!

Use the Arrhenius equation:

ln (k1/k2) = -Ea/R (1/T1 - 1/T2)

Ea = 225 kJ mol-1

k1 = 1.43 x 10-2

T1 = 25°C = 298 K

T2 = 97°C = 370 K

ln (k2/k1) = Ea/R (1/T1 - 1/T2)

ln(k2/0.0143) = ((225 kJ mol-1)/(8.314 J mol-1 K-1)) (1/298 K - 1/370 K)

= (2.706 x 104 K) (0.003356 - .002703) K-1

ln(k2/0.0143) = 17.68

k2/0.0143 = 4.756 x 107

k2 = 6.8 x 105

 

(10) 3. Using any method you like, balance the equation below:

a. MnO4- + Cl- Mn2+ + Cl2 (acidic solution)

 

16 H+ + 2 MnO4- + 10 Cl- 5 Cl2 + 2 Mn2+ + 8 H2O

 MnO4- Mn2+ 2 Cl- Cl2
  MnO4- Mn2+ + H2O 5 x (2 Cl- Cl2 + 2 e-)
   8 H+ + MnO4- Mn2+ + H2O  
 (e- + 8 H+ + MnO4- Mn2+ + H2O) x2  

(15) 4. The mechanism for a hypothetical reaction is:

B X

X + A Q

Q + A C + D

a. Write the net equation for this reaction.

B + 2 A C + D

b. The rate law for this reaction is r = k[B]. Which is the rate determining step in this mechanism?

B X

c. The measured rate of the reaction was -d[A]/dt = 0.020 M sec-1 when [A] = [B] = 0.025 M. what will the rate be if [A] is changed to 0.040 M?

0.020 M sec-

d. The reaction is exergonic. Draw a hypothetical potential energy profile (PE vs rxn coord) for the reaction, using the axis provided. Label completely.

The plot will be similar to the one below. However, note the following:

 


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Last modified 21 February 2011