Department of Chemistry, Geosciences, and Environmental Science
Chemistry 1054-010 College Chemistry Summer I 2005
Exam 4 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 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. According to VSEPR theory, the electron domain geometry associated with four electron domains is:
(a) octahedral (c) linear
(b) tetrahedral (d) trigonal planar
(e) trigonal bipyramidal
Answer: B
2. The hybridization of the orbitals on a central atom is sp2 hybridization. The electron domain geometry associated with this atom is:
(a) octahedral (c) tetrahedral
(b) linear (d) trigonal planar
(e) trigonal bipyramidal
Answer: D
3. How many p bonds are there in the molecule shown above?
(a) 3 (c) 6
(b) 4 (d) 9
(e) 12
Answer: C
4. The bond angle marked “a” in the molecule above is approximately:
(a) 90° (c) 109.5°
(b) 180° (d) 60°
(e) 120°
Answer: E
5. A molecular orbital can accommodate a maximum of ______________ electron(s).
(a) one (c) four
(b) two (d) six
(e) twelve
Answer: 2
6. At constant temperature and pressure, the reaction of 50 mL of N2 with 150 mL of H2 will produce how many milliters (mL’s) of NH3? The reaction is:
N2(g) + 3 H2(g) ¾¾® 2 NH3(g)
(a) 250 mL (c) 200 mL
(b) 50 mL (d) 150 mL
(e) 100 mL
Answer: E
7. Real gases deviate most from ideal gas behavior at:
(a) low temperatures and low pressures. (c) low temperatures and high pressures.
(b) high temperatures and low pressures. (d) high temperatures and high pressures.
Answer: C
8. Which of the following molecules would have hydrogen bonding as the most significant component of its intermolecular forces?
(a) CH4 (c) CH3OH
(b) CH3OCH3 (d) CO2
Answer: C
9. Superconductivity means that
(a) electrons move faster (c) super expensive materials were used
(b) electrons move through a shorter path (d) electrons move without resistance
(e) diamagnetic atoms become elongated.
Answer: D
10. Which point on the phase diagram shown above corresponds to the critical point?
(a) point A (c) point C
(b) point B (d) point D
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) A balloon holds 30.0 kg of helium gas, He(g). What is the volume of the gas, in liters, if the pressure is 124.5 kPa at –22°C? (101.325 kPa = 1 atm)
Given: 30.0 kg of He
___ P = 124.5 kPa
___ T = –22°C
Convert the mass of He into number of moles of He, the pressure into atmospheres, and the temperature to Kelvin:
Solving the ideal gas law for volume:
12. (15 points) (a) Give the electron-domain geometry, the molecular geometry, the ideal bond angles and the hybridization on the central atom for the following Lewis structures:
Answers are italicized in the boxes
(b) One of the molecules above is not polar. Pick which one by indicating the letter of the molecule that you think is not polar.
CCl4 is not polar.
13. (10 points) Concentrated sulfuric acid has a density of 1.84 g/mL and is 95.0% H2SO4 by mass. Calculate the molarity of sulfuric acid (H2SO4(aq)) in this solution.
Assume that you have exactly one liter, or 1000 mL of solution. The mass of 1 L of solution is equal to:
(1000 mL)(1.84 g/mL) = 1840 g
Use the mass percent to calculate the mass of H2SO4 contained in 1 L of solution:
Divide this mass by the molar mass of H2SO4 to calculate the number of moles of H2SO4 in 1 L of solution. This is equal to the molarity:
The molarity is 17.8 M
14. (10 points) Shown below are the molecular orbital energy diagrams for the oxygen molecue, O2, the peroxide ion, O2¯, and the superoxide ion, O22-. Fill each diagram with the appropriate number of electrons and determine the bond order for each species and place your answer in the appropriate spot. Then, determine whether the species is paramagnetic or diamagnetic. Place your answer in the box. Which of these species has the longest bond length and which of these species has the largest bond energy? (Answers: The diagrams are filled with the appropriate number of electrons, the answers are given in the diagram)
The species with the smallest bond order has the longest bond length. This is O22-
The species with the largest bond order has the largest bond energy. This is O2
15. (15 points) Mercury(II)oxide decomposes upon heating to give mercury and oxygen according to the reaction:
2 HgO(s) ¾¾® 2 Hg(l) + O2(g)
An impure sample of mercury(II)oxide is decomposed to produce 583.5 mL of O2(g) that was collected over water at 30°C and a total pressure of 732 torr. How many grams of HgO(s) decomposed?
Known: 583.5 mL of O2 at 30°C and a total pressure of 732 torr
Unknown: ?? g HgO
Convert the known into number of moles of O2(g) using PV = nRT
Since the gas was collected over water, the vapor pressure of water must be subtracted from the total pressure. The vapor pressure of water at 30°C is 31.82 torr (see table below). Converting the pressure to atmospheres, the temperature to Kelving and the volume to liters:
The number of moles of O2 are equal to:
Convert to moles of HgO:
Convert to grams of HgO:
Vapor Pressure of Water at various temperatures
10 points extra credit Please give the complete definition for 2 out of following three terms. You will be given 5 points for each definition. Only answer 2 out of the three, if you answer all 3 only the first two will be graded.
Paramagnetic: any species with unpaired electrons
Supersaturated solution: a solution with a solubility that is greater than the solute’s solubility
Effusion: escape of a gas through a pinhole