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Chemistry 121 The University of British Columbia

Midterm Examination October 27, 2005

Put the first letter of your family/last name in this box.

Time: 60 minutes Family/Last Name (printed):___ANSWERS__________ First Name: ___________________________________ Signature:_____________________________________ Student Number:_______________________________ Please check √ your lecture section: ___101 (MWF 1:00) Dr. Orvig ___102 (MWF 2:00) Dr. Liu ___109 (MWF 9:00) Dr. Sauer ___110 (MWF 10:00) Dr. MacFarlane ___111 (MWF 11:00) Dr. Sauer ___122 (T,Th 2:00) Dr. Wolf ___133 (T,Th 3:30) Dr. Herring ___188 (T,Th 8:00) Dr. Wolf ___199 (T,Th 9:30) Dr. Gates

INSTRUCTIONS

1. Write all answers on this examination paper, and show full details of your solutions for Part 2.

2. Read each question carefully.

3. Check that this examination consists of 10 PAGES PRINTED ON BOTH SIDES,

including a data sheet and periodic table which may be detached for ease of use.

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Marks

Part Question Possible Marks Marks

1 16

2 1 9

2 5

3 4

4 6

5 8

6 4

7 8

Total 60

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Part 1. Multiple Choice (16 marks total) For each numbered statement below, circle the letter that corresponds to the best answer. There is only one correct answer per question. Each question answered correctly is worth 2 marks. 1. Arrange the following in order of increasing electronegativity: Ba, F, As, Cl

(a) As < Cl < Ba < F (b) Cl < As < F < Ba (c) As < F < Ba < Cl (d) Cl < Ba < F < As (e) Ba < As < Cl < F

2. Which combination of quantum numbers (n, l, ml, ms) is NOT allowed?

(a) 3, 2, 1, ½ (b) 4, 2, -3, -½ (b) 3, 2, -2, ½ (c) 3, 1, 0, ½ (d) 4, 3, -1, -½

3. How many angular and radial nodes does a 3p orbital have?

(a) 1 angular and 1 radial (b) 1 angular and 2 radial (c) 2 angular and 1 radial (d) 1 angular and 0 radial (e) 0 angular and 2 radial

4. Which of the following would NOT be expected to exist as an isolable molecule?

(a) NH3 (b) N2H6 (c) SF4 (d) N2H4 (e) none of the above

5. According to the selection rules for electronic spectroscopy, which of the following electronic transitions is forbidden?

(a) 5p→4s (b) 3d→2p (c) 4s→3s (d) 6d→2p (e) none of the above

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6. How many photons of light with a wavelength of 91.2 nm would be required to ionize one mole of hydrogen atoms?

(a) 6.626 × 10-34 (b) 2.18 × 10-18 (c) 42 (d) 6.60 × 1012 (e) 6.02 × 1023

7. Select the pair in which both atoms are paramagnetic in the ground state:

(a) silicon and calcium (b) silver and iron (c) copper and magnesium (d) neon and cobalt (e) fluorine and zinc

8. The binding energy of solid ruthenium (Ru) metal is 455 kJ/mol. When Ru metal is

illuminated by light with a wavelength equal to exactly one half of its photoelectric threshold wavelength, what happens?

(a) no electrons are emitted from the metal (b) electrons are emitted with kinetic energy = 0 J (c) electrons are emitted with kinetic energy = 3.78 × 10-19 J (d) electrons are emitted with kinetic energy = 7.56 × 10-19 J (e) not enough information is given to determine what happens

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Part 2. Short Answer and Written Questions (44 marks total)

9 marks 1. An ion with a +4 charge has two electrons with n = 1, eight electrons with n = 2, and ten electrons with n = 3.

(a) What is the atomic number__24_____ and symbol of the element:______Cr_______ (b) What is the total number of s electrons in this ion:___6__

(c) What is the total number of p electrons in this ion:__12__

(d) What is the total number of d electrons in this ion:___2__

(e) Give the ground state electron configuration and orbital diagram for the neutral atom

from part (a):

Electron configuration Orbital diagram

[Ar]4s13d5

4s 3d

[Ar]

5 marks 2. The longest wavelength of light that causes ionization of the electron from the 4p orbital of an excited hydrogen-like species is 58.4 nm. Give the atomic number of the species in the box below, and use the additional space to show your work.

Answer: Z = 5

The longest wavelength corresponds to the minimum energy required for ionization from the 4p orbital of this species. Calculate the energy from λ:

( )( ) J1040.3m104.58

ms1000.3Js10626.6 189

1834−

−

−−

×=×

××==

λhcE

The minimum ionization energy is calculated as:

J1040.34

10179.2 182

218

4−−

∞ ×=×=−ZEE

Solving for Z:

( ) 25410179.2

1040.3 218

182 =

××

= −

−

Z

Z = 5

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4 marks 3. Draw a 3dxy orbital on the axes below, labelling the axes, nodes and appropriate phases

(signs + or -) of the wave function.

x

y

nodal plane

nodal plane

++ --

6 marks 4. The figure below plots the radial probability distribution 4πr2R2(r) for a particular orbital of

the hydrogen atom. The angular part of the wavefunction for this orbital is:

2/1

41),(

=π

φθY

4πr2R2(r)

(a) Name th (b) What pe

radial no 0% (c) The bou

probabilAngstro

0

0.120.100.080.060.040.02

0

0 5 10 15 r/Angstroms

e orbital to which the above figure refers._3s_____

rcentage of the total radial probability is found between the first and second des. The percentage is closest to: (Circle the best answer)

20% 40% 60% 80% 100%

ndary surface for this orbital can be represented by a sphere such that the ity of finding the electron somewhere within this sphere is 90%. The radius, in ms, of this sphere is closest to: (Circle the best answer)

5 10 15 20

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8 marks 5. Draw Lewis structures for the two best resonance forms of the following species in the box below. Write any non-zero formal charges on the appropriate atoms. Show all lone pairs of electrons as pairs of dots. The central atom is underlined in part (a). Use the space outside the box to show your preliminary work.

Answer:

O C N--1

O C N--1

(a) OCN–

(b) N2O3 (the atom connectivity is O-N-NO2 in this molecule) Answer:

O N N

-1O

O

+1

O N N -1

O

O

+1

4 marks 6. Irradiating fullerene (C60) molecules with laser light having a wavelength of 515 nm results

in diffraction. At what velocity must the C60 molecules travel to possess a wavelength equal to the lasers wavelength?

This is a de Broglie wavelength problem. First calculate the mass of a C60 molecule in kg:

( ) 1-2423

1 molecule kg1020.1g 1000

kg 1molecules10022.6

molgmol 01.1260 −− ×=

××=m

( )( )1-

91-24

34

ms 001.0m10515molecule kg1020.1

)Js10626.6(=

×××

= −−

−

λm=

hv

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8 marks 7. Draw Lewis structures for each of the following molecules. State the molecular shape in each

case. The central atom is underlined. Show all lone pairs of electrons as pairs of dots.

(a) NH2Cl Shape:___trigonal pyramidal_______

Cl

NH H

(b) BrFI2 Shape:___T-shaped______________

F

BrI I