Download 1 Spring 2008 Chemistry 1000 Midterm #1B

Spring 2008
INSTRUCTIONS:
Chemistry 1000 Midterm #1B
____/ 59 marks
1) Please read over the test carefully before beginning. You should have
6 pages of questions, and a formula/periodic table sheet (7 pages total).
2) If your work is not legible, it will be given a mark of zero.
3) Marks will be deducted for incorrect information added to an
otherwise correct answer.
4) Marks will be deducted for improper use of significant figures and for
missing or incorrect units.
5) Show your work for all calculations. Answers without supporting
calculations will not be given full credit.
6) You may use a calculator.
7) You have 90 minutes to complete this test.
1.
(a)
[5 marks]
Which neutral element has the valence electron configuration 4s 2 3d 10 4p 2 ?
[1 marks]
Germanium
(b)
Which neutral element is the first element in the periodic table that has a
completely filled 3 rd shell?
[2 marks]
Copper
1 mark for Zinc
(c)
Give two examples of cations that have the valence electron configuration
3d 5 .
[2 marks]
Fe 3+ , Mn 2+
2.
Complete the table below.
Isotope:
[5 marks]
35
Cl-
135
Ba2+
Number of electrons:
18
54
Number of neutrons:
18
79
Number of protons:
17
56
Overall charge:
-1
+2
1
3.
(a)
[9 marks]
What wavelength of light should you use to excite an electron in a hydrogen
atom from the 3s orbital to the 5s orbital? Express your final answer using an
appropriate SI prefix so that the value is between 0.1 and 1000.
[7 marks]
H has Z = 1
⎛Z2
E n =3 = − R H ⎜⎜ 2
⎝n
⎞
⎛ 12
⎟⎟ = −(2.179 ×10 −18 J )⎜⎜ 2
⎠
⎝3
⎞
⎟⎟ = −2.421×10 −19 J
⎠
⎛Z2
E n =5 = − R H ⎜⎜ 2
⎝n
⎛ 12
⎞
⎟⎟ = −(2.179 × 10 −18 J )⎜⎜ 2
⎝5
⎠
⎞
⎟⎟ = −8.716 × 10 − 20 J
⎠
(
) (
)
E photon = E final − E inital = − 8.716 ×10 −20 J − − 2.421×10 −19 J = 1.550 ×10 −19 J
E=
hc
λ
⎛
8 m⎞
− 34 J ⎞⎛
⎜ 6.626 ×10
⎟⎜ 2.9979 ×10
⎟
hc ⎝
Hz ⎠⎝
s⎠
=
λ=
E
1.550 ×10 −19 J
10 6 µm
λ = 1.282 ×10 −6 m ×
1m
λ = 1.282µm
(b)
How does the wavelength change if the electron is excited from the 3s to the
5p orbital of the hydrogen atom?
[2 marks]
The wavelength of the light needed to excited an electron in a hydrogen atom
from the 4s to the 6 p orbital is the same as that necessary for the excitation
from the 4s to the 6s orbital, since the 6s and 6p orbitals have the same
energy in the hydrogen atom. The energy is only different for multielectron
atoms.
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4.
For each of the following sets of quantum numbers, indicate the maximum
number of electrons allowed in a single atom. If the answer is “none” (i.e. if
there is no such orbital), briefly explain why that is the case.
[6 marks]
(a)
n = 4 l = 3 m l = -2
2
These numbers represent the electrons that occupy one(!) of the seven 4f
orbitals.
(b)
n=2 l=2
none
l can only have a maximum value of n-1.
(c)
n=2
8
The second shell has the 2s and 2p orbitals, in total four orbitals that can be
doubly occupied.
5.
Sketch all the atomic orbitals corresponding to n = 3 and l = 2 in a single
atom. Each sketch should include:
[5 marks]
i.
ii.
a clearly labeled set of axes, and
the label (“name”) of the orbital.
x
x
y
3dxy
y
y
z
3dxz
z
x
y
3dyz
3dx2-y2
3
3dz2.
6.
[4 marks]
[2 mark]
(a) Define the term “first ionization energy”?
energy change of the reaction: A (g) → A + (g) + e - (g)
∆E = I 1
(b) Which of the following elements has the larger first ionization energy,
sodium or magnesium?
[1 mark]
magnesium
(c) Which of the following elements has the larger first ionization energy,
beryllium or boron?
[1 mark]
beryllium
7.
(a)
[8 marks]
Write the set of quantum numbers for each of the valence electrons of Sc.
[3 marks]
three valence electrons:
one electron in 4s: n=4, l= 0, m l = 0, m s = ½
second electron in 4s: n=4, l= 0, m l = 0, m s = -½
one electron in 3d: n=3, l= 2, m l = 0 or +/-1 or +/-2 , m s = ½ or -½
(b)
Which stable ions do you expect to be formed for the elements arsenic,
bromine, scandium?
[3 marks]
As 3- , Br - , Sc 3+
(c)
Place the ions that you suggested in (a) in order of increasing size
smallest _Sc 3+ ____
_Br - ____
4
[2 marks]
_As 3- _____ largest
8.
(a)
[9 marks]
Write the complete electron configuration for a ground state neutral atom of
Zr (zirconium) in the line notation.
[2 marks]
1s 2 2s 2 2p 6 3s 2 3p 2 4s 2 3d 10 4p 6 5s 2 4d 2
(b)
Draw an orbital occupancy diagram showing the valence electrons of a
ground state neutral atom of zirconium. Label the subshells on your diagram.
[3 marks]
5s
(c)
4d
Is a zirconium atom paramagnetic or diamagnetic?
[1 mark]
paramagnetic
(d)
Draw the orbital occupancy diagram for the valence electrons of the Zr 2+
cation.
[2 marks]
5s
(e)
4d
Is the Zr 2+ cation paramagnetic or diamagnetic?
[1 mark]
paramagnetic
9.
(a)
[4 marks]
Calculate the molecular mass for benzene (C6H6).
[1 mark]
molecular mass of C6H6 = 6× 12.011 u+ 6× 1.0079 u= 78.113 u
(b)
Calculate the mass of 0.00525 mol of benzene (in units of g and u).
M(C6H6) = 78.113 g/mol
[3 mark]
m(C6H6) = 0.00525 mol ×78.113 g mol-1 = 0.410 g
one molecule of benzene: m(C6H6) = 78.113 u
0.00525 mol of benzene particles are: 0.00525 mol×6.022 ×1023 mol-1= 3.16 ×1021
particles
mass of 0.00525 mol of benzene: 3.16 ×1021 × 78.1134 u = 2.47 ×1023 u
5
10.
Lithium has two naturally occurring isotopes: 6Li and 7Li. The isotopic masses of 6Li and
Li are 6.015123 and 7.016005 u, respectively. Calculate the natural abundances of these
two isotopes.
[4 marks]
7
m Li = 6.941 u
see periodic table
6
7
Li
6.015123 u
x
Li
7.016005 u
1-x
Li (average)
6.941 u
1
mass
fractional abundance
6.941 u = x × 6.015123 u + (1-x) × 7.016005 u
= x × 6.015123 u + 7.016005 u - x × 7.016005 u
1.000882 u × x = 0.075 u
x
= 0.075
1 – x = 0.925
Therefore the abundance of 6Li is 7.5% and the abundance of 7Li is 92.5%.
6