Chemistry 1b Midterm 2 Worksheet With Answers Page 4
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8Short Answer Questions
Each of the following short answer questions are worth the noted points. Partial credit
will be given. Make sure to show work, report answers to the correct number of
significant figures and use the proper units.
11 pts
1)
A metal forms the salt MCl
. Electrolysis of the molten salt with a current
3
of 0.700 A for 6.63 h produced 3.00 g of the metal. What is the molar mass
of the metal?
Determine the moles of electrons
It
n
F
60
min
60
s
t
. 6
63
h
23900
s
1
h
1
min
It
. 0
700
A
23900
s
n
. 0
173
mol
C
F
96
,
485
mol
3+
Because M forms the compound MCl
the ion that M forms is M
3
3+
-
M
M
+ 3e
Determine the moles of M
1
molM
. 0
173
mole
. 0
0577
molM
3
mole
Determine molar mass M
. 3
00
g
g
52
0 .
mol
. 0
0577
molM
To get full credit for this question you must have shown your work
completely:
4 points for calculating the moles of electrons.
In order to convert between moles of electrons and moles of M you
needed to have an equation that allowed you to get a mole to mole ratio
1
mol
3+
-
3
mol
M. Putting
M
+3e
is not showing your work. I have no
idea where these numbers are coming from or what you are converting
between. Therefore:
3+
3+
3 points for realizing M formed the ion M
(If you did not put M
on
your paper you could only get 4 points even if you got the correct answer.
In order to be able to construct the ½ reaction needed to make the mole to
mole conversion you needed to know the charge on the M)
3+
-
M (If you did not put
2 points for the following equation M
+3e
3+
-
M on your paper you could only get 7 points even if you got
M
+3e
the correct answer because in order to know the mole to mole conversion
factor you needed to know the ½ reaction)
2 points for the correct answer.
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