Chemistry 102
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Dr. Don DeCoste
367J Noyes Lab
decoste@illinois.edu
3-4 pm Tuesdays and Thursdays
By appointment; open door policy
http://chem.illinois.edu
1
To Do
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Continue reading Chapter 3.
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Text homework for Chapter 3.
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Quiz on Friday (1-3.4)
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Lon-Capa assignment #1.
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Consider This…
Does a methane molecule (CH4) contain more
hydrogen or more carbon?
3
Calculate…
Determine the percent mass of oxygen in
each of the following substances:
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H2O (water)
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H2O2 (hydrogen peroxide)
4
Determining a Formula
You have some “nitrogen oxide” and
you want to figure out what it is (both
the formula and the name). You know
it is 30.4% nitrogen by mass.
NxOy = formula?
name?
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Percent Mass Nitrogen in N2O4
2 (14 . 01 )
2 (14 . 01 )  4 (16 . 00 )
x 100%  30.4%
Note: we need the molar mass to
determine the molecular formula.
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A Question For You…
Octane (a constituent of gasoline) is a
hydrocarbon (consists only of carbon and
hydrogen). It has a molar mass of about
114 g/mol and is 84.12% carbon by
mass.
Determine the molecular formula for
octane.
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Clicker Question
Substance AB2 is 42.86% A by mass. Determine
the percent B by mass for substance A2B.
a)
b)
c)
d)
e)
14.28% B
25.00% B
28.57% B
57.14% B
I do not know/I can’t get any of the above
answers.
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Clicker Question
Consider a compound consisting of Co, N, and O
that is 32.21% Co and 15.32% N by mass. Is it
possible to determine the name of the
compound?
a) Yes, and it is cobalt nitrate.
b) Yes, and it is cobalt(I) nitrate.
c) Yes, and it is cobalt(II) nitrate.
d) Yes, and it is cobalt(III) nitrate.
e) Not without a molar mass.
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Formulas for Ionic Compounds
are Empirical Formulas
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Clicker Question
You have separate equal mass samples of each
of the three compounds listed below.
Which sample has the greatest mass of nitrogen?
a)
b)
c)
d)
Ammonium nitrate
Nitrogen monoxide
Magnesium nitride
I do not know.
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Clicker Question
For which of the following is the percent by mass
of carbon equal to the percent by mass of
oxygen?
a) CO
b) C8H15O3
d) C4H6O3
c) C2H4O2
e) CO2
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