Chemical Quantities
Chapter 7 (10)
How many dozen objects are present when
you have:
a) 60 objects: 5 dozen
b) 3 objects: 1/4 dozen or 0.25 dozen
c) 41 objects: 3.4 dozen
d) 2.5 objects: 0.21 dozen
What did you use as the basis for the
conversions?
1 dozen objects = 12 objects
12 objects = 1 dozen
60 objects x 1 dozen = 5 dozen
12 objects
2.5 objects x 1 dozen =
12 objects
.21 dozen
How many objects are present when you
have:
a) 6.0 dozen: 72 objects
b) 3.2 dozen: 38.4 objects
c) 0.025 dozen: 0.30 objects
d) 3.25 x 10-3 dozen: 0.0390 objects
What did you use as the basis for the
conversions?
1 dozen objects = 12 objects
12 objects = 1 dozen
12
objects
6.0 dozen x
= 72 objects
1 dozen
12 objects
0.025 dozen x
= 0.30 objects
1 dozen
Solve these problems:
a) 1.3 x 102 dozen: 1.56 x 103 objects
b) 5.5 objects: 0.458 dozen
c) 2.5 x 10-4 objects: 2.08 x 10-5 dozen
d) 7.4 x 10-1 dozen: 8.88 objects
If each object weighs 10 grams, how many
objects do you have when you have 90
grams worth?
1
object
90 g x
= 9 objects
10 g
If each object weighs 10 grams, how many
grams of objects do you have when you
have 3.0 dozen objects?
3.0 dozen
12 objects 10 g = 360 g
1 dozen 1 object
Solve these problems:
a) How many dozen objects are present
when you have 100. g of objects? Each
dozen weighs 32 g.
100. g
1 dozen
32 g
= 3.13 dozen
Solve these problems:
b) How many objects are present when you
have 100. g of objects? Each dozen
weighs 32 g.
100. g
1 dozen 12 objects
= 37.5 objects
32 g
1 dozen
Solve these problems:
c) How much do 1.25 dozen objects weigh
if each dozen weighs 32 g?
1.25 doz
32 g
1 dozen
= 40.0 grams
I. The Mole Concept
A. Just as “1 dozen = 12”, chemistry uses
this ratio:
1 mole = 6.02 x
23
10
B. This number is frequently referred to as:
Avogadro’s Number
1) How many atoms of carbon are present
in 0.250 moles of carbon?
0.250 mol
6.02 x 1023 atoms
1 mole
= 1.51 x 1023 atoms
2) How many moles of carbon atoms are
present in 5.00 x 1010 atoms of carbon?
5.00 x 1010 atoms
1 mole
6.02 x 1023 atoms
= 8.31 x 10-14 mol
3) The weight of 1 mole of an element (6.02 x
1023 atoms of that element) = it’s atomic
weight
a) What is the mass of 0.330 mol of carbon?
0.330 mole C
12.011 g
1 mole
= 3.96 g
b) How many moles are present in 1.00 x 102
grams of carbon?
100. g 1 mole
= 8.33 mole
12.011 g
c) How many atoms of carbon are present in
2.00 grams of carbon?
2.00 g 1 mole
12.011 g
6.02 x 1023 atoms
1 mole
= 1.00 e23
d) How much do 3.0 x 109 atoms of carbon
weigh?
3.0 x 109 atoms
1 mole
12.011 g
6.02 x 1023 atoms 1 mole
= 6.0 x 10-14 g
e) How many grams do 0.125 moles of H2O
weigh?
0.125 mol H2O
18 g
= 2.25 g
1 mole H2O
f) How many molecules of water are present
in 90. grams of water?
90. g H2O 1 mole H2O 6.02 x 1023 molec
18 g
1 mole H2O
= 3.0 x 1024 molecules
II. Percent Composition
A. Percent = Part x 100
whole
Ex: What is the percent boys in a class
made up of 5 boys and 15 girls?
% boys = 5 boys x 100
20 total
= 25% boys
Ex: What is the percent oxygen in H2O, by
weight?
% oxygen = 16 g oxygen x 100
18 g total
= 88.9% oxygen
III. Empirical and Molecular
Formulas
A. The subscripts in a chemical formula are
the mole ratios of atoms
Ex: H2O has 2 moles of hydrogen : 1 mole of
oxygen in every 1 mole of water
Ex: C6H12O6 has 6 moles of carbon : 12 mole
of hydrogen: 6 moles of oxygen in every 1
mole of C6H12O6
B. If the percent composition is known, the
molar ratio of each element can be
calculated:
Ex: A sample is known to consist of 88.9% O
and 11.1% H. What is the formula of this
compound?
Div by Div by
a.w.
smallest
88.9 O
11.1 H
5.56
1
11.1
2
H2O
Ex: A sample is known to consist of 40.0% C
and 6.70% H and the rest is oxygen. What
is the empirical formula of this compound?
If the molecular weight is 180 g, what is the
molecular formula?
Div by Div by
a.w.
smallest
40.0 C
6.70 H
53.3 O
3.30
1
6.70
2
3.30
1
CH2O
Empirical Formula
(Smallest whole
number ratio)
Molecular Formula = C6H12O6