Tarleton State University

Department of Chemistry, Geosciences, and Environmental Science

 

          Chemistry 1054-010                        College Chemistry                                  Fall 2005

 

                                                                     Exam 2-A-Key

 

Name:_______________________________________________________

                              (Please write your name legibly)

 

Read all directions and questions carefully!!  This exam consists of two parts.  The first part consists of 10 multiple choice questions worth four points each for a total of 40 points.  The second part consists of five numerical problems worth either 10 or 15 points per question for a total of 60 points.  Show all your work necessary for the numerical problems as partial credit will be given for those problems.

 

Possibly Useful Constants

 

            Avogadro’s Number:  NA = 6.022 × 1023 particles/mol

            q = (heat capacity)×∆T

            q = (mass in grams)×(specific heat)×∆T

            q = (number of moles)×(molar heat capacity)×∆T

            Calorimetry:  qrxn = –Ccalorimeter×∆T  or

                                    qrxn = –(mass of solution)×(specific heat of solution)×∆T

 

 

Score

 

                                    Part 1 (40 points):_____________________

 

 

                                    Part 2 (60 points):_____________________

 

 

                                    Total (100 points):_____________________

 

 

Don’t forget to put your name on this test!

 

Good Luck!!

Part 1

 

Multiple Choice

 

Please indicate the answer to each question by putting your choice in the space provided.  There is only one correct answer for each question.  There will be 10 multiple choice questions worth 4 points each.

 

1.  Which one of the following species is a strong electrolyte?

      (a)  HF                                                                  (c)  HCl

      (b)  C6H12O6                                                          (d)  CCl4

                                                (e)  NH3

 

Answer:  C

 

2.  Which one of the following salts will be insoluble in water?

      (a)  Na3PO4                                                           (c)  NH4Cl

      (b)  Al(NO3)3                                                        (d)  FeSO4

                                                (e)  CaCO3

 

Answer:  E

 

3.  In which of the following species does sulfur have an oxidation number of +6?

      (a)  S8                                                                    (c)  H2S

      (b)  H2SO3                                                             (d)  H2SO4

                                                (e)  H2S2O3

 

Answer:  D

 

4.  Use the attached activity series to predict which one of the following combinations will result in a displacement reaction:

      (a)  Zn(s)  +  Al(NO3)3(aq)                                    (c)  Co(s)  +  MgCl2(aq)

      (b)  Cu(s)  +  HCl(aq)                                           (d)  Mn(s)  +  FeCl2(aq)

 

Answer:  D

 

5.  The DE for a system that absorbs 12.4 J of heat and does 4.2 J of work on the surroundings is:

      (a)  16.6 J                                                              (d)  12.4 J

      (b)  4.2 J                                                                (e)  –16.6 J

      (c)  –8.2 J                                                              (f)  8.2 J

 

Answer:  F

6.  Given the data below, DH° for the reaction

3 Cl2(g)  +  PH3(g) ¾¾® PCl3(g)  +  3 HCl(g)

      is equal to:

      (a)  –570.37 kJ                                                      (c)  570.37 kJ

      (b)  –385.77 kJ                                                      (d)  385.77 kJ

            (e)  The DHf° for Cl2(g) is needed for this calculation.

 

 

Compound

DHf° (kJ/mol)

 

 

PCl3(g)

–288.07

 

 

HCl(g)

–92.30

 

 

PH3(g)

5.3

 

 

Answer:  A

 

7.  Of the substances below, which is not considered a fossil fuel?

      (a)  coal                                                                 (c)  natural gas

      (b)  petroleum                                                       (d)  gasoline

                                                (e)  hydrogen

 

Answer:  E

 

8.  Which of the following types of food has the highest fuel value?

      (a)  carbohydrates                                                 (c)  proteins

      (b)  fats

 

Answer:  B

 

9.  What volume of a 6.00 M NaOH solution must be diluted to prepare 200.0 mL of a 0.880 M NaOH solution?

      (a)  2.64 mL                                                          (c)  176 mL

      (b)  29.3 mL                                                          (d)  26.4 mL

                                                (e)  50.0 mL

 

Answer:  B

Dilution problem: 

 

10.  What is the molarity of sulfate ions in a 0.15 M Al2(SO4)3 solution?

      (a)  0.15 M                                                            (c)  0.30 M

      (b)  0.45 M                                                            (d)  0.050 M

                                                (e)  0.075 M

 

Answer:  B

 

Since there are 3 SO42- ions per formula unit:  Molarity of sulfate = 0.15 M × 3 = 0.45 M

 

Part 2

 

Numerical Problems

 

Solve the following problems, keeping track of significant figures where applicable.  Please show all the work necessary to obtain your answer in order to receive partial credit for possibly wrong answers.  Generally, full credit will not be given for the correct answer without any of the work performed to obtain the answer being shown on the paper.  Each question is worth either 10 or 15 points.

 

11.  (10 points)  How many grams of solute are contained in 232.7 mL of a 0.4478 M C6H12O6 solution?

 

Using the definition of molarity, the molarity times the volume of the solution in liters will give you the number of moles of C6H12O6:

Convert from moles of C6H12O6 to grams of C6H12O6:

12.  (15 points)  In a titration experiment, a student determined the concentration of a NaOH solution by titrating it against 25.00 mL of 0.3155 M H2SO4.  If it required 29.34 mL of the NaOH solution to reach the endpoint of the titration, what is the molarity of the NaOH solution?  The neutralization reaction is:

2 NaOH(aq)  +  H2SO4(aq)  ¾¾®  Na2SO4(aq) +  2 H2O(l)

 

This is a solution stoichiometry problem.  Identify the known and unknown quantities:

known:  25.00 mL of 0.3155 M H2SO4

unknown:  29.34 mL of ?? M NaOH

Calculate the number of moles of the known quantity (H2SO4):

Convert moles of H2SO4 into moles of NaOH using the balanced chemical reaction:

Calculate the molarity of a solution that contains 0.01578 mol of NaOH in 29.34 mL of solution:

13.  (10 points)  A 1.28 g sample of vanillin, C8H8O3, is combusted in a bomb calorimeter with a heat capacity of 7.58 kJ/°C.  During the combustion, the temperature of the calorimeter increased from 24.50°C to 28.75°C.  What is the heat of combustion of 1.28 g of vanillin and the heat of combustion per mole of vanillin?

 

In a calorimeter experiment:

qrxn = –Ccalorimeter×∆T

The heat capacity of the calorimeter is given in the problem:  Ccalorimeter = 7.58 kJ/°C

 

The temperature difference can be calculated:  DT = 28.75°C – 24.50°C = 4.25°C

 

The heat of combustion is equal to:  qrxn = –(7.58 kJ/°C)(4.25°C)= –30.9 kJ

 

The heat of combustion per mole is equal to:

 

 

14.  (10 points)  Write the net ionic equations for the following molecular equations:

 

(a)  Al(OH)3(s)  +  3 HI(aq)  ¾¾®  AlI3(aq)  +  3 H2O(l)

 

The total ionic equation is:

Al(OH)3(s) + 3 H+(aq)  +  3 I¯(aq)  ¾¾®  Al3+(aq)  +  3 I¯(aq)  +  3 H2O(l)

Removing the spectator ion, I¯, the net ionic equation is:

Al(OH)3(s)  +  3 H+(aq)  ¾¾®  Al3+(aq)  +  3 H2O(l)

 

 

(b)  Mg(s)  +  SnSO4(aq)  ¾¾®  MgSO4(aq)  +  Sn(s)

The total ionic equation is:

Mg(s)  +  Sn2+(aq)  +  SO42-(aq)  ¾¾®  Mg2+(aq)  +  SO42-(aq)  +  Sn(s)

Removing the spectator ion, SO42-, the net ionic equation is:

Mg(s)  +  Sn2+(aq)  ¾¾®  Mg2+(aq)  +  Sn(s)

 

 

(c)  Zn(s)  +  2 HCl(aq)  ¾¾®  ZnCl2(aq)  +  H2(g)

The total ionic equation is:

Zn(s)  +  2 H+(aq)  +  2 Cl¯(aq)  ¾¾®  Zn2+(aq)  +  2 Cl¯(aq)  +  H2(g)

Removing the spectator ion, Cl¯, the net ionic equation is:

Zn(s)  +  2 H+(aq)  ¾¾®  Zn2+(aq)  +  H2(g)

 

15.  (15 points)  Elemental phosphorus, P4(s), reacts vigorously with bromine according to the reaction:

P4(s)  + 6 Br2(l) ¾¾®  4 PBr3(s)

___ What is the maximum amount, in grams, of PBr3 produced from the reaction of 61.94 g of P4 and 150.8 g of Br2?

 

Knowns:  61.94 g of P4 and 150.8 g of Br2

 

Unknown:  ?? g of PBr3

 

This is a limiting reactant problem.  Calculate how much PBr3 each reactant can make:

The smaller amount of PBr3 is the amount produced.  Therefore, 0.6291 mol of PBr3 is produced.  Converting from moles to grams:

(10 points extra credit)  Give the complete definition for 2 out of the 3 terms below.  5 points will be given for each correct complete definition.  Clearly note which 2 definitions you want graded, if you write three definitions without a clear notation, only the first two will be graded.

 

Reduction:  the loss of electrons by a substance

 

 

 

 

Heat capacity: the amount of heat required to raise the temperature of an object by 1°C of 1 K.

 

 

 

 

Endothermic reaction: a reaction that absorbs heat.