Unit 3 Exam-Solutions

Version 227 – Unit 3 Exam – vandenbout – (51335) 1

This print-out should have 27 questions.Multiple-choice questions may continue onthe next column or page – find all choicesbefore answering.

001 10.0 pointsWhich compound should be most viscous?

1. H C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

H

2.

O

H

C

H

H

C

H

H

C

H

O

H

C

H

H

C

H

H

C

H

H

C

H

O

H

H

correct

3. O

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

O

H

4. H C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

O

H

Explanation:You would predict that the most viscous

compound exhibits the greatest amount ofintermolecular forces, and thus contains themost O H functional groups (the more -OHgroups, the more hydrogen bonding).

002 10.0 pointsSelect the FALSE statement about com-pounds with octahedral geometries.

1. The central atom in compounds with oc-tahedral geometries always obey the octetrule. correct

2. Compounds with octahedral geometriesdo not have eight atoms bonded to the centralatom.

3. Octahedral electronic geometries mayhave four atoms bonded to the central atom.

4. If a compound has a square planar molec-ular geometry, it corresponds to an octahedralelectronic geometry.

5. Angles in compounds with octahedralmolecular geometries are typically 90 and 180degrees.

6. Octahedral molecular geometries alwaysrequire sp3d2 hybridizations.

Explanation:Compounds with octahedral geometries

have expanded valence shells. These are ex-ceptions to the octet rule.

003 10.0 points

This question is merely a placeholder forthe points in the hand-graded portion of theexam.

1. yes CORRECT2. sure3. without a doubt4. absolutely5. do I have a choice?

Explanation:

This question is merely a placeholder forthe points in the hand-graded portion of theexam.

004 10.0 pointsSO2−

4is more/less polar than SO2−

3because

1. more; the tetrahedral geometry increasesthe polarity of S O bonds

2. more; it has four polar S O bondsrather than three polar S O bonds.

3. less; the four polar S O bonds in


SO2−4

are symmetrical and cancel the dipolemoments. correct

4. less; it is a resonance structure.

5. more; the four polar S O bonds in

SO2−4

are arranged non-symmetrically leavinga net dipole moment.

Explanation:SO2−

4has no lone pairs on the central S

atom. It has tetrahedral molecular geometry.Each bond angle is 109.5◦. The dipole mo-ments cancel and the molecule is non-polar.SO2−

3has one lone pair on the central S

atom. It has pyramidal molecular geometry.The bond angles are not equal so the dipolemoments cannot cancel. This molecule ispolar.

005 10.0 points

s p p p ??? p

The hybridized orbital marked ??? is ofwhat type?

1. sp2 correct

2. sp3d

3. sp3

4. sp

Explanation:To make the hybrid orbital, the s orbital

and only 2 of the 3 available p orbitals werehybridized, as evidenced by the fact that oneof the p orbitals appears not to have beenhybridized. The hybrid orbital thus producedwould be sp2.

006 (part 1 of 3) 10.0 pointsPart 1: Consider the line structure givenbelow:

N

OH

a

How many carbons are in this structure?

1. seven correct

2. five

3. four

4. six

5. eight

Explanation:The carbons are all implied at the ends of

the bonds (lines) and also where there areangles (bends) in those bond lines.

007 (part 2 of 3) 10.0 pointsPart 2: What is the hybridization of theatom show at the position labeled a ?

1. sp

2. sp3d2

3. sp2 correct

4. sp3

5. sp3d

Explanation:There is a double, a single, and one more

implied single bond to a hydrogen which is 3total electron regions which matches up withan sp2 hybridization.

008 (part 3 of 3) 10.0 pointsPart 3: Even though there are no lone pairsshown on this structure, how many lone pairsare actually there ?

1. four pairs

2. two pairs

3. one pair

4. none


5. three pairs correct

Explanation:The nitrogen needs one pair and the oxygen

needs two pairs to give 3 total lone pairs inthe structure.

009 10.0 pointsFor which of the following is the type of solidINCORRECTLY given?

1. C6H6, molecular solid

2. KF, ionic solid

3. H2O, molecular solid

4. Ni, metallic solid

5. CsI, covalent solid correct

Explanation:CsI is an ionic solid. An ionic bond is a

bond between a cation(

Cs+1)

and an anion(

I−1)

.

010 10.0 pointsA substance has a melting point of 1200 K. Itconducts electricity while melting but not inthe solid. This substance would be classifiedas

1. a metallic solid.

2. an ionic solid. correct

3. a covalent network solid.

4. a hydrogen bonded solid.

5. a molecular solid.

Explanation:Ionic solids are held together by electro-

static attractions between cations and anions.In a melted phase, the ions can move andtherefore can carry current.

011 10.0 pointsConsider the molecule CF4.

1. It has four non-polar bonds and is a non-polar molecule.

2. It has four polar bonds and is a polarmolecule.

3. It has four anti-bonds and is a polarbear.

4. It has four non-polar bonds and is a polarmolecule.

5. It has four polar bonds and is a non-polarmolecule. correct

Explanation:Fluorine has a very high electronegativity

when compared to carbon. A covalent bondbetween two molecules with such differentelectronegativities is polar. The moleculargeometry of the molecule CF4 is tetrahedral,and all the bonds have an equal dipole mo-ment since they are all between the sameatoms (C and F). The net dipole of themolecule is zero since all of the individualdipoles cancel each other out. A moleculewith no net dipole moment is non-polar.

012 10.0 pointsIf a molecule of AB2 has 2 lone pairs of elec-trons on A, what would you expect the B-A-Bangle to be closest to?

1. 62.5 degrees

2. 109.5 degrees correct

3. 90 degrees

4. 120 degrees

Explanation:This molecule fits the general formula

AB2U2. These structures, which have 4 re-gions of high electron density, have tetra-hedral electronic geometries, and they havebond angles close to 109.5 degrees.

013 10.0 points


In the molecule

C C

CH3

CH3

H3C

H3C

the six carbon atoms (do, do not) lie in thesame plane, and the bond angle at a double-bonded carbon atom is approximately howmany degrees?

1. do not; 90◦.

2. do; 180◦.

3. do; 90◦.

4. do not; 180◦.

5. do; 120◦. correct

6. do not; 109◦.

7. do; 109◦.

8. do not; 120◦.

Explanation:

014 10.0 pointsWhat structural feature makes unsaturatedfats less rigid than saturated fats?

1. Unsaturated fats have weaker covalentbonds along the chain; weaker covalent bondsresult in reduced rigidity.

2.Unsaturated fats have a bend that reducesthe strength and number of dispersion forcesbetween molecules. correct

3. The statement is untrue; saturated fatsare more rigid because their C-C bond lengthsare shorter and stronger than for unsaturatedfats.

4. The statement is untrue; saturated fatsare more rigid because of their higher molec-ular weight.

Explanation:

Unsaturated fats have at least one doublebond. Along the fatty acid chain, there isa bend at each double bond. (Imagine anelbow.) Packing of such fats is less denseat the molecular level, and fewer dispersionforces are possible. This results in a less rigidsubstance.

015 10.0 points

The bonding in metals like tin and leadis best described as (localized, delocalized)whereas the bonding in insulators or semicon-ductors (such as diamond or silicon) is bestdescribed as (localized, delocalized).

1. delocalized; localized CORRECT2. localized; localized3. delocalized; delocalized4. localized; delocalized

Explanation:

Metallic solids are solids composed of metalatoms that are held together by metallicbonds. These bonds are like huge molecu-lar orbitals that span across the whole solid.Electrons are not just held between a few ofatoms in a sigma bond. Rather, there is a seaof electrons everywhere. In covalent, molecu-lar, and ionic solids, electrons are localized inbonds between atoms.

016 10.0 points

Use VSEPR theory to predict the shape ofthe molecule SeF6.

1. linear

2. None of these

3. bent or angular

4. trigonal-pyramidal

5. tetrahedral

6. trigonal-bipyramidal

7. octahedral correct


8. trigonal-planar

Explanation:geometry of SeF6 = ?The Lewis structure for SeF6 is

Se Fb

b

bb

bb

F

b

bbb

bb

F

b

b

bb

bb

Fb

b

b b

b b

F

b

b bb

bb

F

b

b

bb

bb

The molecule is an exception to the octetrule because Se has an expanded octet (12 e−

in this case). The six areas of high electrondensity make the molecular geometry octahe-dral.

Se

F

F

F

F

F

F

017 10.0 pointsWhich of the following are important contri-butions that MO theory makes to chemistry.

I. The ability to use MO theory with acomputer to calculate the minimum energygeometry of a molecule.

II. The ability to predict the energy atwhich a molecule will absorb light.

III. The ability to predict whether or not amolecule should be paramagnetic or diamag-netic.

1. only I

2. I and II

3. I, II, and III correct

4. only II

5. none of them

6. II and III

Explanation:The MO theory is a powerful theory that,

based on Quantum mechanical calculations,and with the help of a powerful computer, canvery quickly calculate the energies of electronsin a small or large molecule as the relative po-sitions of the atoms of the molecule are movedrelative to each other. This iterative calcula-tion results in the lowest energy arrangementof all the atoms and all the electrons of theatoms in the molecule, thus leading to the pre-diction of the most likely arrangement of theatoms and electrons. Therefore, the shape,magnetism (pair or unpaired electrons resid-ing in orbitals), and the energy difference ofthe HOMO and LUMO can be calculated.

018 10.0 pointsWhich of the following combinations of hy-bridization and molecular geometry is notpossible?

1. sp3; tetrahedral

2. sp2; angular

3. sp3d; linear

4. sp3d2; octahedral

5. sp3d2; angular correct

6. sp3; angular

Explanation:Angular molecular geometries can only be

formed from sp2 and sp3 hybridizations.

019 10.0 pointsA molecule with tetrahedral geometry mustbe nonpolar.

1. False correct

2. True

Explanation:

020 10.0 pointsWhich molecular geometries can stem from


tetrahedral electronic geometry?

1. trigonal pyramidal, seesaw

2. tetrahedral, trigonal bipyramidal, linear,angular, seesaw

3. tetrahedral, trigonal pyramidal, angularcorrect

4. only tetrahedral

5. tetrahedral, T-shaped, linear

Explanation:Tetrahedral, trigonal pyramidal and angu-

lar are three molecular geometries that canstem from tetrahedral electronic geometry.

021 10.0 pointsWhich of the followingmolecules is incorrectlymatched with molecular geometry?

1. NH3 : tetrahedral correct

2. H2O : angular

3. AsF5 : trigonal bipyramidal

4. TeF6 : octahedral

5. SiF4 : tetrahedral

Explanation:The electronic geometry of NH3 is tetrahe-

dral, but its molecular geometry is trigonalpyramidal.

022 10.0 pointsWhich represents a false statement?

1. All molecules with four regions of electrondensity have sp3 hybridization.

2.The sp2 hybridization corresponds to trig-onal planar electronic arrangements.

3. All square planar molecules have octahe-dral electronic arrangements.

4. All angular molecules are sp2 hybridized.correct

Explanation:Angular molecules can be sp2 or sp3 hy-

bridized.

023 10.0 pointsWhat are all of the intermolecular forces thatare responsible for the existence of ice?

1. dipole-dipole and London forces

2. dipole-dipole and ion-ion

3. hydrogen bonding and dipole-dipole

4. London forces

5. dipole-dipole, London forces, and hydro-gen bonding correct

Explanation:Ice is solid H2O. This molecule exhibits

dispersion. Because it is polar it also exhibitsdipole-dipole interactions. Finally because ofthe H bonded to the electronegative O atomit also exhibits hydrogen bonding; this polarbond can orient and interact with the lonepair of an O atom on another H2O molecule.

024 10.0 points

Examine the structural formula for aspirin.

CCH

3O

O

HO

C

O

12

3

Each number identifies a different func-tional group within the molecule. Whichfunctional group includes only sp2 hybridizedcarbon atoms?

1. functional groups 2 and 3 only

2. functional groups 1 and 2 only correct


3. functional group 2 only


5. functional groups 1, 2, and 3

6. functional groups 1 and 3 only


Explanation:The benzene ring contains 6 carbon atoms

all of which are sp2. The carboxylic acidcontains a sp2 C . However, Group 3 alsoincludes a methyl group which is sp3.

025 10.0 pointsWhich of the following molecules would beexpected to have the lowest boiling point?

1. C2H6

2. C12H26

3. C8H18

4. CH4 correct

5. C16H34

Explanation:These molecules are all very similar in that

they are made up only of hydrogen and car-bon. They are all covalently bonded, nonpo-lar, and cannot form hydrogen bonds. There-fore the only intermolecular force that at-tracts these molecules to each other are dis-persion forces. Dispersion forces are generallystronger for molecules that have more elec-trons or are larger, so you would predict thatthe smallest molecule in this group (CH4)would have the weakest intermolecular forcesand thus would have the lowest boiling point.

026 10.0 pointsA molecule with a central atom and twobonded atoms is bent with a bond angle of117◦. Assuming the central atom satisfies theoctet rule, how many lone pairs does it have?

1. 3 lone pairs

2. 1 lone pair correct

3. 2 lone pairs

4. 0 lone pairs

5. 4 lone pairs

Explanation:Only a molecule with two singly bonded

atoms and one lone pair (a pair of non-bonding electrons) would satisfy all of theconstraints laid out in the problem, most no-tably the bond angle which implies trigonalplanar geometry and thus 3 regions of highelectron density (RHED). Since two of theseare atoms, the third must be a lone pair. Ex-ample: NOH.

027 10.0 pointsWhich of the following has the highest vaporpressure?

1. CH3CH2OH

2. CH3CH2COOH

3. CH3CH2OCH2CH3 correct

4. CH3OH

5. H2O

Explanation:The compound with the highest vapor

pressure will be the one that most readilyevaporates (the one with the weakest inter-molecular forces). All of the choices ex-hibit London (van der Waals) forces andare also polar, so they can exhibit dipole-dipole interactions. However, unlike the otherchoices, CH3CH2OCH2CH3 does not haveO H bonds and so cannot exhibit hydro-gen bonding. Although CH3CH2OCH2CH3

is the largest molecule, the higher Londonforces are not strong enough to counteractthe lack of hydrogen bonding so overall it has


the weakest intermolecular forces.

Unit 3 Exam-Solutions

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