Tarleton State University

Department of Chemistry, Geosciences, and Environmental Science

 

          Chemistry 1054-010                         College Chemistry                                  Fall 2009

 

                                                                     Exam 1-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

 

 

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.  A general law that summarizes how nature behaves is a(n)

      (a)  hypothesis                                                             (c)  theory

      (b)  law                                                                        (d)  experiment

                                    (e)  test

 

Answer:  B  (See “A Closer Look:  The Scientific Method page 13 in your book)

 

2.  Which of the following are physical processes?

      1.  Rusting of a nail

      2.  Freezing of water

      3.  Decomposition of water into its elements

      4.  Compression of oxygen gas

 

      (a)  Item 4 only                                                           (c)  Items 3 and 4

      (b)  Items 2 and 4                                                        (d)  Item 2 only

                                    (e)  Items 2 and 3

 

Answer:  B

 

3.  Accuracy refers to:

(a)  how close a measured number is to other measured numbers.

(b)  how close a measured number is to zero.

(c)  how close a measured number is to the true value.

(d)  how close a measured number is to the calculated value.

(e)  how close a measured number is to infinity.

 

Answer:  C

 

4.  Rutherford’s gold foil experiment is credited with what result?

      (a)  the discovery of the electron.

      (b)  the charge of the electron.

      (c)  the atomic theory of matter.

      (d)  the nuclear model of the atom.

 

Answer:  D

 


 

5.  55Mn2+ has ________ protons, _______ neutrons, and _______ electrons.

      (a)  25, 55, 23                                                              (c)  25, 30, 27

      (b)  25, 30, 25                                                              (d)  25, 30, 23

                                    (e)  25, 25, 23

 

Answer:  D

 

6.  Which of the following compounds is molecular?

      (a)  BaCl2                                                                    (c)  CCl4

      (b)   KCl                                                                      (d)  AlCl3

                                    (e)  All the compounds above are ionic.

 

Answer:  C

Molecular compounds contain nonmetals only.

 

7.  The definition of the mole is based on which isotope?

      (a)  1H                                                                          (c)  4He

      (b)  12C                                                                        (d)  16O

 

Answer:  B

 

8.  Which of the following reactions is a combustion reaction?

(a)  N2(g)  +  3 H2(g)  ¾¾®  2 NH3(g)

(b)  C6H12O6(s)  +  6 O2(g)  ¾¾®  6 CO2(g)  +  6 H2O(l)

(c)  2 KClO3(s)  ¾¾®  2 KCl(s)  +  3 O2(g)

(d)  2 Na(s)  +  2 H2O(l)  ¾¾®  2 NaOH(aq)  +  H2(g)

 

Answer:  B

 

9.  The empirical formula of a compound is CH2O.  If the compound’s molar mass is 90.1 g/mol, what is the molecular formula of the compound?

      (a)  CH2O                                                                    (c)  C3H6O3

      (b)  C2H4O2                                                                 (d)  C4H8O4

(e)  The molecular formula cannot be determined without more information.

 

Answer:  C

The empirical formula weight for CH2O is equal to 30.03 g/mol.  Divide this into the molar mass to get that the molecular formula is 3 times the empirical formula, C3H6O3

 

10.  Given the following unbalanced reaction:  Mg3N2  +  H2SO4  ¾¾®  MgSO4  +  (NH4)2SO4 

      What is the coefficient in front of H2SO4 when the equation is balanced with whole number coefficients?

      (a)  1                                                                            (c)  3

      (b)  2                                                                            (d)  4

 

Answer:  D

The balanced reaction is:  Mg3N2  +  4 H2SO4  ¾¾®  3 MgSO4  + (NH4)2SO4

 

 

 

 

 


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)  Perform the following unit conversions:

 

(a)  (3)  325.2 mg to g

 

  (4 sig figs)

You would not have been given the conversion between mg and g.  You should know this one.

 

 

 

(b)  (3)  25 L to gal  (1 gal = 3.7854 L)

 

   (2 sig figs)

 

 

(c)  (4)  0.027 ft3 to in3

Because you are dealing with cubic lengths, you must first cube the conversion between ft and in:

(12 in)3 = (1 ft)3

1728 in3 = 1 ft3

  (2 sig figs)

 

 

 

12.  (15 points)  A compound containing only carbon, hydrogen, and oxygen contains 39.34% C, 8.25% H,  and 52.41% O by mass.  The molar mass of the compound is 183.2 g/mol.  What are the empirical and molecular formulas for this compound?

 

Assuming 100 grams of compound:

 

 

The empirical formula is C2H5O2.  It has an empirical formula weight of 61.06 g/mol (= 2 × 12.01 + 5 × 1.008 + 2 × 16.00).  Divide this into the molar mass:

The molecular formula is 3 times the empirical formula which is C6H15O6.

 

 

 

 

 

13.  (10 points)  What is the volume occupied by 37 g of platinum?  The density of platinum is equal to 21.45 g/mL.

 

  (2 sig figs)

 

 

14.  (10 points)  How many moles are there in 23.50 g of Al(NO3)3?

23.50 g of Al(NO3)3

??? mol of Al(NO3)3

 

Calculate molar mass of Al(NO3)3:

                                                                 1 Al:  26.98

                                                 3 N:  3×14.01 = 42.03

                                                 9 O:  9×16.00 = 144.0

                                                                           213.0

Convert from grams to moles:

 (4 sig figs)

 

 

 

 

15.  (15 points)  Hydrogen sulfide is an impurity found in natural gas.  It is removed via the following reaction:

8 H2S(g)  +  4 O2(g)  ¾¾®  S8(l)  +  8 H2O(g)

Suppose that 50. g of H2S and 25 g of O2 are allowed to fully react.

 

(a)  (10)  What is the maximum amount (in grams) of S8 produced?

 

Limiting reactant problem.  Calculate the amount of S8 that each reactant could make assuming that it fully reacts:

 

 

H2S yields the smaller amount of S8.  Therefore, it is the limiting reactant and 0.19 mol of S8 are produced.  Converting to grams of S8:

  (2 sig figs)

 

 

(b)  (5)  Suppose that when the reaction was performed, 43 g of S8 was collected.  What is the %yield for this reaction?

 

  (2 sig figs)


 

(10 points extra credit)  Define 2 out the 3 following terms.  Credit will be given only for the complete definition.  No partial credit will be given.  Choose only 2 out of the three terms.  If you give definitions for all three terms without clearly noting which ones you want graded, only the first two terms will be graded.

 

Law of constant composition:  the composition of a substance is always the same no matter where the substance originates.

 

 

 

 

 

Mass number:  the number of protons plus the number of neutrons in a particular atom.

 

 

 

 

 

Atomic mass unit: