1
PRACTICE FINAL EXAM: CHEM 20A1, 2007:
SOLUTIONS
1.
A mixture of iron (Fe) and aluminum (Al) weighs 9.62 g.
The sample is reacted with an excess of
HCl and produces 0.738 g of H
2
according to the reactions
2 Al + 6 HCl = 2 AlCl
3
+ 3 H
2
Fe + 2 HCl = FeCl
2
+ H
2
How much iron was present in the original mixture?
Let x be the number of grams of Fe in the original mixture; then (9.62x) is the number of grams
of Al originally present.
For every mole of Al that reacts, 3/2 mole of H
2
are produced; and for every mole of Fe that
reacts, 1 mole of H
2
is produced.
The total number of moles of H
2
produced is
0.738
g H
2
2.02
g H
2
/
mole H
2
= 0.365 mole H
2
, from which it
follows that
3
2
(9.62

x
)
27.0
+
x
55.8
=
0.365
.
(The atomic weights of Al and Fe are 27.0 and 55.8.)
Solving for x gives x = 4.50 g.
2.
Consider the ionic diatomic LM whose dipole moment is 8.0 debye (D) and whose bond length is
2.02A.
What is the percent ionic character of this bond?
1 D = 3.336 x 10
30
Cm, implying 8.0 D = 2.67 x 10
29
Cm.
The relationship between dipole moment (
μ
) and bond length (R) is
μ
= qR, where q is the
magnitude of the charge that is separated, i.e., charges of +q and –q are separated by a distance
R.
Solving for q we have q =
μ
/R = (2.67 x 10
29
Cm)/(2.02 x 10
10
m) = 1.32 x 10
19
C.
Thus the fraction of a fundamental charge (e = 1.60 x 10
19
C) that is separated is
(1.32 x 10
19
Cm)/(1.60 x 10
19
m) = 0.826, implying a percent ionic character of 82.6.
3.
Consider two hydrogen nuclei (protons), separated by 1A along the zaxis; suppose one is at the
origin, with coordinates (x, y, z) = (0, 0, 0), and the other is at the point (0, 0, 1A).
a.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview.
Sign up
to
access the rest of the document.
 Fall '05
 Scerri
 Atom, Electron, Reaction, Electric charge, Molecule, percent ionic character

Click to edit the document details