This preview shows pages 1–5. Sign up to view the full content.
ENSC 3313, Fall 2003
TEST 1
Name
________________________________
Answer all 10 questions, 2 points each
.
1.
The element Cf
254
is sold by the Atomic Energy Commission for $10/microgram (one millionth of a
gram) and has a halflife of 55 days. Please consider a onemilligram sample. If the sample is prepared
and shipped via overnight delivery, what is the value of the material that decays during the 24 hours of
shipping?
λ
−
=
t
o
e
A
A
λ
−
=
t
A
A
ln
o
()
day
35
.
79
55
5
.
0
ln
=
λ
⇒
λ
−
=
g
5
.
987
e
g
1000
A
35
.
79
1
µ
=
µ
=
−
125
$
g
10
$
g
5
.
987
1000
=
µ
×
µ
−
2.
CdS can be used as a light sensor due to its absorption of impingent photons and the ensuing creation of
electronhole pairs.
If the sensor "sees" light with wavelengths < 500 nm (i.e., green light), estimate the
band gap energy of this material.
Note:
the speed of light is c = 3
×
10
8
m/s and Plank's constant is
h = 4.13
×
10
15
eV
⋅
s.
The energy of a photon is given by:
eV
48
.
2
10
500
10
3
10
13
.
4
c
h
h
E
9
8
15
=
×
×
⋅
×
=
λ
=
ν
=
−
−
Since only photons with enough energy to promote an electron from the valence to conduction
band are absorbed, this is approximately equal to the CdS band gap energy.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document 3.
If we had 2 SiO
4
tetrahedra sharing an edge, as sketched, what would be the chemical formula and
charge on the unit (Si
x
O
y
)
n–
. ( what are the values of x, y and n?)
There would be an Si
4+
at the center of each tetrahedra
(
•
).
Two corners are shared – there are 6 oxygen (O) total;
two share with 2 silicon, hence 4 of oxygen lack one
bond, hence we have 4
–
. Hence (Si
2
O
6
)
4–
.
4.
Identify 6 planes of symmetry in a regular tetrahedron.
Draw a regular tetrahedron in a cube, thence draw a cube face diagonal, of
which there are 6. The cube face diagonals correspond to a tetrahedron edge.
5.
Show
by calculation that NaCl is a 6fold coordination compound. The ionic radii are Na
+
= 0.102 nm,
Cl
–
= 0.181 nm.
Sketched is the top face of the 6fold coordination cell, which is cubic.
2r + 2R = a = 2(0.102 + 0.181) = 0.566 nm
But the large ions must not
touch along the face diagonal.
Hence,
( )
( )
,
800
.
0
724
.
0
,
566
.
0
2
181
.
0
4
,
a
2
R
4
<
<
<
☺
6.
Identify the plane and direction shown.
Origin at 0
Extrapolate ‘x’ axis
Intercepts are
1
2
1
1
Take reciprocals; plane
= (121)
Direction =
1
2
1
0
Clear fraction =
[ ]
2
01
Note
direction lies in plane and
0
2
1
1
2
0
2
=
⋅
+
⋅
+
⋅
,
☺
.
7.
Calculate the packing factor of CsCl. The ionic radii are Cs = 0.169 nm and Cl = 0.181 nm. Recollect
that CsCl does not
have the same structure as NaCl (or LiF)!
cell
of
Volume
ions
of
Volume
PF
=
One kind of ion is at the cell corner, the other at the cell center.
There is, on average, one of each type per cell.
They contact along the cube diagonal, so
a
3
r
2
R
2
=
+
()
68
.
0
3
181
.
0
2
169
.
0
2
181
.
0
169
.
0
3
4
.
1
PF
3
3
3
=
+
+
π
=
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document8.
The maximum solubility of tin in a CuSn bronze is 15.8 w/o at 580
°
C.
To what a/o Sn does this
correspond?
The amu are 63.54 and 118.69 respectively.
This is the end of the preview. Sign up
to
access the rest of the document.
This note was uploaded on 01/10/2012 for the course ENSC 3313 taught by Professor Staff during the Fall '11 term at Oklahoma State.
 Fall '11
 staff

Click to edit the document details