Section 6.1: Team Learning Worksheet
Name ____________
1. Why can’t we just count atoms directly instead of counting them by weighing them?
Why must we count atoms at all? That is, why is it important to know how many
atoms we have in a sample?
2. The average mass of a carbon atom is 12.011. Assuming you could pick up one carbon
atom, what is the likelihood that you would randomly get one with a mass of 12.011?
Explain.
3. Chlorine exists mainly as two isotopes, 37Cl and 35Cl. Which is more abundant? How
do you know?
Section 6.2: Team Learning Worksheet
1. Explain the difference between the terms “atomic mass” and “molar mass.”
2. A 0.821-mol sample of a substance composed of diatomic molecules (of the same
atom) has a mass of 131.3 g. Identify this molecule.
3. How many molecules of water are there in a 5.00-g sample of water? How many
hydrogen atoms are there in this sample?
4. Consider separate samples of carbon dioxide and ammonia, each with the same mass.
Which sample contains the greater number of molecules? How many times greater?
Answers
6.1
1. Atoms are too small to count directly, and even if we could see them, there are too many in a small
sample to count. In order to know the formula for a compound, the relative numbers of each type of
atom in the compound must be known.
2. There is a 0% chance of finding a carbon atom with a mass of 12.011. The number 12.011 is an average
mass of carbon isotopes, and no carbon atom has that mass.
3. The 35Cl isotope is more abundant. The average atomic mass for chlorine given on the periodic table is
35.45. The value 35.45 is closer to 35 than 37.
6.2
1. The term “atomic mass” refers to the mass of one atom, while the term “molar mass” refers to the mass
of 1 mol of a substance (either an element or a compound). The atomic masses listed on the periodic
table are actually average atomic masses. The units for atomic masses are amu’s, and the units for
molar masses are grams (thus, when we say that the molar mass of water, for example, is 18.016 g, we
are saying that 1 mol of water has a mass of 18.016 g).
2. The molecule is Br2. Since mass and mol values are given, the molar mass of the substance could be
calculated as follows:
131.3 g
= 159.9 g/mol
0.821 mol
Because the molecule is diatomic (with the same atoms), we know that the formula of the molecule
must be X2. Thus the mass of each atom is (159.9/2) = 79.95, which is the atomic mass of bromine.
3. There are 1.67 × 1023 molecules of water and 3.34 × 1023 hydrogen atoms in 5.00 g of water.
5.00 g H2O 
6.022 x 10 23 molecules H 2 O
1 mole H 2 O

18.016 g H 2 O
1 mole H 2 O
1.67  1023 molecules H2O 
=1.67  1023 molecules H2O
2 atoms hydrogen
= 3.34  1023 hydrogen atoms
1 molecule H 2 O
4. The sample of ammonia contains the greater number of molecules, by a factor of 2.58. A lower molar
mass means that a molecule of ammonia has a lower mass than a molecule of carbon dioxide. In order
to have equal masses of each, there must be a greater number of ammonia molecules than carbon
dioxide molecules. To determine the factor assume any mass, determine the number of moles, and
divide. For example, assume we have 100.0 g of each:
1 mole CO 2
= 2.27 mol CO2
44.01 g CO 2
1 mole NH3
100.0 g NH3 
= 5.87 mol NH3
17.034 g NH3
100.0 g CO2 
5.87
44.01
= 2.58 =
2.27
17.034