Lesson 37:
Chemical Compounds,
Parallelograms
In chemistry courses some problems
deal with the weight relationships of
chemical compounds. Most of these
problems are straightforward ratio
and percent problems, and it is
necessary to know only two things
about chemistry to be able to work
them.
The first thing is that atoms always
combine in the same combinations
when they unite to form a specific
compound. The chemical formulas
for the compounds tell us the number
of each kind of atom in a molecule of
the compound.
For example
H2 O: formula for water. Each
molecule of water has two
hydrogen and one oxygen atom.
NaOH: formula for sodium
hydroxide. Each molecule has one
sodium (Na), one oxygen (O), and
one hydrogen (H).
The second thing we need to know is
that every kind of atom has a different
weight. We express the weights of atoms
in grams. The gram atomic weight of
oxygen is 16 grams and the gram atomic
weight for hydrogen is 1 gram. We will
give the gram atomic weights of the
elements in parentheses. In a problem
about oxygen and hydrogen, we would
note the gram atomic weights as
(O, 16; H, 1)
When elements combine to form a
compound, we say that the molecules of
the compound have a gram molecular
weight that can be found from the gram
atomic weights. Since the formula for
water is
H 2O
We find that the gram molecular weight
of water is
2(1 gram) + 1(16 grams) = 18 grams
Example:
The chemical formula for water is
H O.2 if we have 3600 grams of
water, what is the weight of the
oxygen (O, 16: H, 1)?
Answer:
Gram molecular weight of water =
18 grams
O/3600 = 16/18
O = 3200 grams
Example:
The chemical formula for ammonia is
NH . This
tells us that each ammonia
3
molecule contains one nitrogen atom
and three hydrogen atoms. If we
have 510 grams of ammonia, how
much does the nitrogen weigh (H, 1:
N, 14)?
Answer:
14(1) + 1(3) = 17
N/510 = 14/17
N = 420 grams
Example:
The formula for ammonium chloride
is NH Cl. This means
that in one
4
molecule of ammonium chloride there
is one atom of nitrogen, four atoms of
hydrogen, and one atom of chlorine.
How many grams of chlorine are
there in 1060 grams of ammonium
chloride (N, 14: H, 1: Cl, 35)?
Answer:
1(14) + 4(1) + 1(35) = 53
Cl/1060 = 35/53
Cl = 700 grams
Parallelograms have four special
properties.
1. The pairs of parallel sides have
equal lengths (are congruent)
2. Angles opposite each other have
equal measures (are congruent)
3. The sum of any two adjacent angles
is 180 degrees
4. The diagonals bisect each other
Example:
ABCD is a parallelogram and
m<BAD is 65°. Find m<ADC and
m<DCB. Find x and y.
B
C
y+7
7
x+2
A
4
D
Answer:
m<DCB = 65°
m<ADC = 115°
y = -3
x=5
Example:
ABCD is a parallelogram. Find x
and y.
A
B
(10x + 50)°
(5x + 10)° (y + 10)°
D
C
Answer:
x=8
y = 120
Homework: Lesson 37 #1-30