Chem Hughbanks Exam 2, March 10, 2016

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1 Chem Hughbanks Exam 2, March 10, 2016 Name (Print) UIN # Section 502 Exam 2, On the last page of this exam, you ve been given a periodic table and some physical constants. You ll probably want to tear that page off to use during the exam you don t need to turn it in with the rest of the exam. The exam contains 11 problems, with 7 numbered pages. You have 75 minutes to complete the exam. Please show ALL your work as clearly as possible this will help us award you partial credit if appropriate. Even correct answers without supporting work may not receive credit. You may use an approved calculator for the exam, one without extensive programmable capabilities or the ability to store alphanumeric information. Print your name above, provide your UIN number, and sign the honor code statement below. On my honor as an Aggie, I will neither give nor receive unauthorized assistance on this exam. SIGNATURE:

2 For questions 1 through 5, consider two identical glass bulbs, each having a volume of 1.00 L. The bulbs are evacuated, and then one is filled with oxygen and the other with neon. When filled, each bulb is at a pressure of 500 torr and a temperature of 50 C. Assume that the gases behave ideally. (Note that you should be able to answer these without doing any significant calculations.) (1) (3 pts) Which bulb contains fewer moles of gas? (Circle your answer.) Oxygen Neon both the zd same (2) (3 pts) In which bulb is the average kinetic energy of the molecules higher? (Circle your answer.) Oxygen Neon both the zd same (For grading) Scores 1 /3 2 /3 3 /3 4 /3 5 /4 6 /12 7 /7 8 /20 9 /10 10 /12 11 /23 Tot. /100 (3) (3 pts) In which bulb is the density (g/cm 3 ) of the gas lower? (Circle your answer.) Oxygen Neon zd both the same (4) (3 pts) In which bulb is the average molecular speed lower? (Circle your answer.) Oxygen zd Neon both the same (5) (4 pts) If one of these bulbs were opened in this room, would the net flow of gas be out of or into the bulb? (Circle your answer.) Why? (2 pts) The net flow of gas would be out of the bulb. into zd the bulb. (2 pts) Give a brief explanation: Gas will flow from area of higher pressure to an area of lower pressure. The pressure inside the bulb (500 torr) is less than atmospheric pressure (760 torr). Thus, the net flow of gas will be into the bulb. 1

3 (6) (12 pts) For the electronic configuration listed in each case, write G for an atom in its ground state, write E for an atom in an excited state, or write I for an impossible configuration. If the configuration is possible, give the symbol for the corresponding neutral atom (leave it blank if you put an I in the first space). G, E, or I Symbol of element (if possible) (a) 1s 2 2s 2 G Be (b) 1s 2 2s 2 2p 3 G N (c) [Ne]3s 2 3p 3 4f 4 I (d) 1s 2 2s 2 2p 4 3s 2 E Ne (e) [Ne]3s 2 3p 3 4s 2 E Cl (f) [Ne]3s 2 3p 6 4s 3 3d 2 I (7) (7 pts) In a selenium atom, which electron is most effective in screening a 3p electron from the nucleus, (Circle one of the five choices below.) a 3s electron a 4p electron a 1s electron another 3p electron all the above are all equally effective 2

4 (8) (20 points) In (a) (d), please circle your choices. In (e) and (f), writing clearly, fill in the blanks with the choices given. BE SURE TO READ EACH STATEMENT VERY CAREFULLY! (a) (3 pts) Which one of the following elements is most likely to form a cation with a +2 charge? Br Mg Rb Cl Al S (b) (3 pts) Which one of the following compounds would be best classified as ionic? CH 3 NH 2 SrF 2 BrCl CO 2 C 2 H 6 GaP (c) (3 pts) Which one of the following atoms or ions would have the largest radius? Cs + Ba + Ba Rb + Xe I (d) (3 pts) All of the following atoms and ions are paramagnetic in their ground states. Which one would be most strongly paramagnetic (i.e., have the largest magnetic moment)? Mg + Mn 2+ Co 3+ N O (e) (4 pts) Rank the following in order of decreasing ionization energy: Ar, Ca 2+, Cl, K, S 2 (largest) Ca 2+ > Ar > Cl - > S 2- >K (smallest) (f) (4 pts) Rank the following in order of increasing size: Al, C, Na, Ne, Mg 2+ (smallest) Ne < Mg 2+ < C < Al < Na (largest) 3

5 Questions 9-11 show all work (45 pts) (9) (10 points) You work in a semiconductor production plant which uses several chlorofluorocarbons in its manufacturing process. One day you find an unlabeled gas cylinder, and you are assigned to figure out what is in the tank. First you fill a 1.0 L high-pressure container with the gas. At a pressure of atm. and a temperature of 25 C, you determine that the mass of the gas in the container is g. Then you send a sample of the gas to an outside lab for elemental analysis, and they report that the gas contains 14.05% C, 44.46% F, and 41.48% Cl by mass. Find the molecular formula of this gas. (5.000 atm)( kpa/atm) = kpa PV=nRT n= PV RT = ( kpa)(1.0 L) (8.314 L kpa K -1 mol -1 )( K) g molar mass of gas = mol For a 100 grams of gas: g C mol C =1.17 mol C g g F mol F =2.34 mol F g Cl 1.17 mol C mol F mol Cl g mol Cl g =1 mol C =2 mol F =1 mol Cl =1.17 mol Cl = mol =170.9 g/mol (12.00 g/mol)(1 mol C) + (19.00 g/mol)(2 mol F) + (35.45 g/mol)(1 mol Cl) = g per mole of gas But the molar mass found above is g/mol /85.45 = 2 Double the moles of each atom: (12.00 g/mol)(2 mol C) + (19.00 g/mol)(4 mol F) + (35.45 g/mol)(2 mol Cl) = g mole of gas Therefore, the molecular formula for the gas is C 2 F 4 Cl 2 4

6 (10) (12 points) The energy levels of the helium atom are important in the operation of helium-neon lasers. If a helium aom absorbs a photon with a wavelength of nm, one electron will be promoted from the 1s orbital to a 2p orbital. (a) (5 pts) Find the energy, in J, gained by a helium atom undergoing this transition nm 10-9 m nm E= hc λ = 58.43x10-9 m J s ( m s -1 ) = ( m) E = J (b) (7 pts) The ionization energy of helium is 2372 kj/mol. Suppose that a helium atom has already undergone the transition above, so that it has one electron in a 2p level. What is the longest wavelength of light that would be capable of supplying enough energy to eject an electron from this excited helium atom, producing a He + ion? (HINT: It may help to draw an energy level diagram for this process.) Convert ionization energy to J/atom: 2372 kj mol mol 6.023x10 23 atoms 1000 J kj =3.938x10-18 J/atom The energy needed to eject the electron from one excited He atom is the difference in the ionization energy and the energy gained from the excitation to the 2p level: E= 3.938x10-18 J J = J Convert to wavelength: λ= hc E = (6.626x10-34 J s)(2.998x10 8 m s -1 ) J = 3.690x10-7 m 3.690x10-7 m 109 nm m = 3.690x10 2 nm λ = nm 5

7 (11) (23 points) (a) (7 pts) Draw the best possible Lewis structure for nitryl fluoride, NO 2 F, below. Show all important resonance structures if any exist. (Nitrogen is the central atom.) O O N N F O F O Answer the following questions, consistent with your Lewis diagram(s) (a) (2 pts) In the space provided, give the nitrogen atom formal charge in NO 2 F: 1+ (b) (2 pts) In the space provided, give the nitrogen atom hybridization NO 2 F: sp 2 (c) (2 pts) Let S be the sum of all three angles (in degrees) around the nitrogen ( ONO plus both ONF angles). Circle the correct statement from the six choices below: S > 540 S = < S < 540 S = < S < 360 S = 180 6

8 (11) continued (d) (10 pts) Nitric acid, HNO 3, can be written as NO 2 (OH) to highlight its structural relationship to NO 2 F. The two molecules are the same except nitric acid has an OH group where NO 2 F has a fluorine atom. Likewise, the two molecules have closely related Lewis diagrams. Use Lewis diagrams for three additional species, NO +, NNO, and NO + 2, to answer the questions below. If you can t assign an exact value to a bond order, give the best estimate you can by using inequalities (for example, if you think a bond order is greater than 2 but less than 2.5, write 2 < B.O. < 2.5). + a = NO bond (or average) in NO 2 b = the N OH bond in HNO 3 (not the N O bond to oxygen without hydrogen) c = NO bond in NNO d = NO bond in NO + e = NO bond (or average) in NO 2 F O N O O N N N O Bond Bond order Bond distance* a b c d e *Bond distance choices are (in pm): 106.2, 115.2, 121.8, 122.8,

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