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CHEM 103 Module 2 Chemistry Exam
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Module 22.1: MOLECULAR WEIGHT
A compound is made up of two or more elements combined in a definite ratio
that is represented by a molecular formula. Each of these elements has a
certain atomic weight, which can be found in the periodic table. The sum of
the atomic weights of the atoms in the molecular formula is called
the formula weight or molecular weight or formula mass. We will use
the term molecular weight for this number in our work. Here are three
examples of molecular weight determination:
Molecular Weight
Ca3(PO4)2 3 Ca = 3 x 40.08 = 120.24
=
2 P = 2 x 30.97 = 61.94
8 O = 8 x 16.00 = 128.00
310.18
Molecular Weight
C3H5O2Cl 3 C = 3 x 12.01 = 36.03
=
5 H = 5 x 1.008 = 5.04
2 O = 2 x 16.00 = 32.00
1 Cl = 1 x 35.45 = 35.45
108.52
Molecular Weight
Al2(SO4)3 2 Al = 2 x 26.98 = 53.96
=
3S = 3 x 32.07 = 96.21
12 x
12 O = = 192.00
16.00
2.2: MOLES
,Chemical compounds react with one another in amounts that are based on
their molecular weights; this chemically reactive amount of compound
is called a mole.
The number of moles of a substance is equivalent to a certain weight in
grams, and it can be calculated by the following equation.
moles = grams / molecular weight
A chemical compound's weight in grams is equivalent to a certain number of
moles of the substance, and this number can be calculated with the following
equation.
grams = moles x molecular weight
Calculate the number of moles in 10.0 grams of each of the following
compounds:
10.0 g ÷ 310.18 = 0.0322 mol (3 sig fig because of 10.0
Ca3(PO4)2
g)
C3H5O2Cl 10.0 g ÷ 108.52 = 0.0921 mol
Al2(SO4)3 10.0 g ÷ 342.17 = 0.0292 mol
Calculate the number of grams of each in 0.0500 mol of the following
compounds:
Ca3(PO4)2 0.0500 mol x 310.18 = 15.5 g
C3H5O2Cl 0.0500 mol x 108.52 = 5.43 g
Al2(SO4)3 0.0500 mol x 342.17 = 17.1 g
2.3: PERCENT COMPOSITION
The molecular formula represents the definite ratio of elements in a
compound. The weight of each element present in the compound represents
a certain percentage of the total weight of the compound. The percentage of
each element present in a compound is called the % composition of the
compound.
The percentage of an element present in a compound can be calculated as
shown below:
, The % composition calculation for several compounds is shown
below:
= % Ca = (120.24 ÷ 310.18) x
Ca3(PO4)2 3 Ca = 3 x 40.08
120.24 100 = 38.76%
%P = (61.94 ÷ 310.18) x
2P = 2 x 30.97 = 61.94
100 = 19.97%
= 128.0 %O = (128.00 ÷ 310.18) x
8O = 8 x 16.00
0 100 = 41.27%
310.18
%C = (36.03 ÷ 108.52) x 100
C3H5O2Cl 3 C = 3 x 12.01 = 36.03
= 33.20%
%H = (5.04 ÷ 108.52) x 100
5 H = 5 x 1.008 = 5.04
= 4.64%
%O = (32.00 ÷ 108.52) x 100
2 O = 2 x 16.00 = 32.00
= 29.49%
%Cl = (35.45 ÷ 108.52) x 100
1 Cl = 1 x 35.45 = 35.45
= 32.67%
108.52
%Al = (53.96 ÷ 342.17) x
Al2(SO4)3 2 Al = 2 x 26.98 = 53.96
100 = 15.77%
%S = (96.21 ÷ 342.17) x
3 S = 3 x 32.07 = 96.21
100 = 28.12%
= 192.0 %O = (192.00 ÷ 342.17) x
2 O = 12 x 16.00
0 100 = 56.11%
342.17
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