Tarleton State University

Department of Chemistry, Geosciences, and Environmental Science

 

          Chemistry 1054-010                         College Chemistry                               Spring 2010

 

                                                                     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.  If a sample of matter is uniform throughout and cannot be separated into other substances by physical processes, but can be decomposed into other substances by chemical processes, it is called a(n) __________________.

 

      (a)  heterogeneous mixture                                          (c)  homogeneous mixture

      (b)  element                                                                 (d)  compound

                                    (e)  mixture of elements

 

Answer:  D

 

2.  How many significant figures should be in the answer to the following computation:

      (a)  1                                                                            (c)  3

      (b)  2                                                                            (d)  4

                                    (e)  5

 

Answer:  B

The subtraction of 11.13 – 2.6 = 8.5 is cut off after the tenth’s digit which limits the computation to 2 significant figures.

 

3.  The SI unit of volume is the ____________.

      (a)  mL                                                                        (c)  L

      (b)  gal                                                                         (d)  cm3

                                    (e)  m3

 

Answer:  E

A volume is a length cubed.  The SI unit of length is the meter.  So the SI unit of volume is the cubic meter (m3)

 

4.  The nucleus of an atom contains ______________.

      (a)  electrons                                                                (c)  protons and neutrons

      (b)  protons, neutrons, and electrons                           (d)  protons and electrons

                                    (e)  protons only

 

Answer:  C

 

5.  Which of the following experiments proved the nuclear model of the atom?

      (a)  Thomson’s cathode ray tube experiment              (c)  Rutherford’s gold foil experiment

      (b)  Millikan’s oil drop experiment                             (d)  Dalton’s atomic theory

 

Answer:  C

 

 

 

6.  The correct name for CaH2 is _______________________.

      (a)  hydrocalcium                                                        (c)  calcium hydroxide

      (b)  calcium dihydride                                                 (d)  calcium dihydroxide

                                    (e)  calcium hydride

 

Answer:  E

Ionic compounds:   name the cation, then the anion.  There are no prefixes denoting relative number of each ion.

 

7.  Which of the following compounds is ionic?

      (a)  KCl                                                                       (c)  HCl

      (b)  PCl5                                                                      (d)  CCl4

                                    (e)  All of the above compounds are ionic.

 

Answer:  A

Ionic compounds contain metals and nonmetals.  Molecular compounds contain nonmetals only. 

 

8.  Of the reactions below, which one is a decomposition reaction?

      (a)  2 Mg +  O2  ¾¾®  2 MgO                                (c)  N2 + 3 H2  ¾¾®  2 NH3

      (b)  2 CH4  +  4 O2  ¾¾®  2 CO2 + 4 H2O              (d)  NH4Cl  ¾¾®  NH3 + HCl

                                    (e)  Cd(NO3)2  +  Na2S  ¾¾®  CdS  +  2 NaNO3

 

Answer:  D

 

9.  Calculate the percent by mass of nitrogen in PtCl2(NH3)2.

      (a)  4.67                                                                       (c)  9.90

      (b)  9.34                                                                       (d)  4.95

                                    (e)  12.67

 

Answer:  B

 

10.  The definition of the atomic mass unit (amu) is based on which of the following isotopes?

      (a)  16O                                                                        (c)  1H

      (b)  4He                                                                        (d)  12C

 

Answer:  D

 

 

 

 

 

 

 

 

 

 

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)  Fill in the blanks in the following table with the appropriate information:

 

 

Symbol

Number of protons

38

52

49

Number of neutrons

49 (=87 –38)

75

66 (=115 – 49)

Number of electrons

36 (= 38 – (+2))

54

49 (= number of protons)

Net charge

+ 2 (or 2+)

2 (=52 – 54)

0

 


 

12.  (15 points)  A certain compound contains 55.80% C, 7.03%H, and 37.17%O by mass.  It has a molar mass of 172.2 g/mol.  What are the empirical and molecular formulas for this compound?

 

Assuming 100 grams of compound, the empirical formula can be calculated as follows:

The empirical formula is C2H3O.  The empirical formula weight is 43.04 amu.  Divide this into the molar mass to get the multiplication factor for the molecular formula:

The molecular formula is 4 times the empirical formula which is C8H12O4

 

 

 

 

13.  (10 points)  Rhenium is a dense metal with a density of 21.02 g/mL at 20°C.  What is the volume occupied by 1255 g of rhenium?

 

Given:  density = 21.02 g/mL; mass = 1255 g

Unknown:  volume = ?

 

Using the definition of density (=mass/volume):

 (4 sig figs)

14.  (10 points)  How many grams are there in 3.75 mol of SO2?

 

Calculate the molar mass (or formula weight of SO­2):

1 S × 32.07 + 2 O × 16.00 = 64.07 g/mol

Converting from moles to grams:

 

 

 

 

 

15. (15 points)  Diborane (B2H6) can be prepared by the following reaction:

3 NaBH4(s)  +  4 BF3(g)  ¾¾®  2 B2H6(g)  + 3 NaBF4(s)

What is the maximum amount, in grams, of B2H6 produced when 2.25 g of NaBH4 is reacted with 4.75 g of BF3?

Limiting reactant problem!!

Given:  2.25 g NaBH4 and 4.75 g of BF3

Unknown:  ?? g of B2H6

 

Calculate how much B2H6 that each reactant could make assuming it fully reacts:

Since BF3 produces the smaller amount of B2H6, it is the limiting reactant.  That means that 0.0355 mol of B2H6 is produced.  Converting from moles to grams:

 


 

 

(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:  a pure substance will have the same relative composition no matter where it comes from.

 

 

 

 

 

Mass number:  the number of protons plus the number of neutrons.

 

 

 

 

 

Atomic mass unit:  exactly 1/12 the mass of a carbon-12 atom.