Department of Chemistry, Geosciences and Environmental Sciences

 

                                                                     Exam 4-B-Key

                       

Chemisty 1054:  Section 010                                                                                       Fall 2005

 

Name:________________________________________________________

 

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 and Equations

 

Gas constant:  R = 0.08206 L·atm/mol·K                  1 atm = 760 torr, exactly

 

Ideal Gas Law:  PV = nRT

 

 

                   

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.     The molecular geometry associated with four bonding electron domains and one nonbonding electron domains is:

(a) trigonal bipyramidal                                              (c)  seesaw

(b) linear                                                                     (d)  T-shaped

                                    (e)  trigonal pyramidal

 

Answer:  C

 

2.  The hydridization associated with a trigonal bipyramidal electron domain geometry is:

      (a)  sp                                                                          (c)  d2sp3

      (b)  dsp3                                                                      (d)  sp2                             

                                          (e)  sp3

 

Answer:  B

 

3.  A bonding molecular orbital __________________ the corresponding antibonding molecular orbital derived from the same pair of atomic orbitals.

      (a)  is always lower in energy than                             (c)  can accommodate more electrons than

      (b)  is always degenerate with                                    (d)  can accommodate less electrons than

                                          (e)  is always higher in energy than

 

Answer:  A

 

4NH3(g)  +  5 O2(g)  ¾®  4 NO(g)  +  6 H2O(g)

3.  Given the balanced gas phase reaction above, 24.00 mL of NH3(g) will react fully to produce how many milliliters of H2O(g) at the same temperature and pressure?

      (a)  24.00 mL                                                              (c)  36.00 mL

      (b)  18.00 mL                                                              (d)  unable to determine without T and P

 

Answer:  C

 

Using Gay-Lussac’s law: 

 

4.  Which of the following is a correct mathematical statement for Boyle’s Law?

      (a)  PV = constant                                                        (c) 

      (b)                                                         (d)  V = constant ´ n

                                          (e)  V = constant ´ P

 

Answer:  A

 

5.  Which one of the following gases has the smallest density at the same temperature and pressure?

      (a)  HCl                                                                       (c)  Ar

      (b)  F2                                                                          (d)  H2S

            (e)  All of these gases have the same density at the same temperature and pressure.

 

Answer:  D

 

The gas with the smallest molar mass has the smallest density.

 

6.  Gases deviate the least from the ideal gas law at:

      (a)  high temperatures and high pressures                   (c)  low temperatures and high pressures.

      (b)  low temperatures and low pressures                    (d)  high temperatures and low pressures.

 

Answer:  D

 

7.  What is the strongest type of interparticle forces are found in KCl(aq)?

      (a)  ionic bonding                                                        (c)  dipole-dipole forces

      (b)  London dispersion forces                                     (d)  hydrogen bonding

                                          (e)  ion-dipole forces

 

Answer:  E

 

8.  The point at which all three phases co-exist is called the ________________.

      (a)  boiling point                                                         (c)  triple point

      (b)  critical point                                                         (d)  sublimation point

 

Answer:  C

 

9.  Which type of liquid crystalline phase is used to generate colors on liquid crystal displays?

      (a)  smectic                                                                 (c)  cholestic

      (b)  nematic

 

Answer:  C

 

10.  Which of the following expressions gives the definition for molarity?

      (a)                                                     (c) 

      (b)                                           (d) 

 

Answer:  C

 

                                                                                                                                   


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 NH3(g) are contained in a container of volume 175.0 mL at a temperature of 55°C and a pressure of 743 torr?

 

Before using the ideal gas law, perform the necessary conversions:

Calculate the number of moles of NH3(g) by solving the ideal gas law for n:

Convert from moles of NH3 to moles of NH3:

Moles of NH3 = (6.56 × 10-3 mol)(17.03 g/mol) = 0.112 g NH3  (3 sig figs)


12.  (15 points)  Give the electron domain geometry, the molecular geometry, the approximate bond angles and the hybridization on the central atom given the following Lewis structures below:

 

 


13.  (10 points)  What is the mole fraction and percent by mass of NaCl in a solution prepared by dissolving 8.80 g of NaCl in 42.20 g of H2O? 

 

The definition of percent by mass is: 

 

The mole fraction of NaCl is defined as the moles of NaCl divided by the total number of moles in the solution.  Calculating the moles of NaCl and H2O in solution:

The mole fraction of NaCl is equal to:

 


14.  (10 points)  Given the molecular orbital energy level diagrams shown below, determine the bond orders for the molecule C2 and for the ion C22+ by filling each diagram with the correct number of valence electrons (electrons are represented by up and down arrows).  State whether each species is diamagnetic or paramagnetic.  From the bond orders, state which species has the shorter bond length and which one has the smaller bond energy.

 

C2 has a total of 8 valence electrons (4 from each carbon atom.)  C22+ has two less valence electrons for a total of 6 valence electrons.  Filling the molecular orbital diagrams below is represents by up and down arrow in the diagrams:

For C2, the bond order is equal to ½×(8–4) = 2.  For C22+, the bond order is equal to ½ ×(6–4) = 1.

 

C2 has no unpaired electrons so it is diamagnetic.  C22+ has two unpaired electrons so it is paramagnetic.

 

The species with the larger bond order has the shorter bond length.  This is C2.

The species with the smaller bond order has the smaller bond energy.  This is C22+.


 

15.  (15 points)  Calcium hydride reacts with water to produce calcium hydroxide and hydrogen gas according to the reaction:

CaH2(s)  + 2 H2O(l)  ¾¾®  Ca(OH)2(aq)  +  2 H2(g)

       This reaction is used to inflate life rafts or weather balloons.  What would be the pressure, in atm,  of H2(g) generated in a weather balloon of volume 45.0 L at a temperature of 12°C by the decomposition of 65.0 g of CaH2(s)?

 

Gas Stoichiometry problem:

 

Known:  65.0 g of CaH2             Unknown:  ?? atm of H2(g) at 12°C and a volume of 45.0 L

 

Calculate moles of CaH2:

Convert to moles of H2 using the balanced chemical reaction:

Use the ideal gas law to calculate the pressure of 3.08 mol of H2 contained in a volume of 45.0 L and a temperature of 12°C:

T = 12°C + 273 = 385 K


(10 points extra credit)  Give the complete definition for 2 out of the following 3 terms.  You will be given 5 points for each complete correct definition.  Clearly note which 2 answers you want graded.  If you give 3 definitions with no indication of which ones you want graded, only the first two definitions will be graded.

 

Surface tension:  a measure of the tendency of a liquid to maintain a minimum surface area.

 

 

 

 

 

Diffusion:  the spread of one gas into another gas.

 

 

 

 

 

Thermoplastic:  a plastic that can be shaped through the application of heat and pressure and then can be reshaped by reheating.