Section 6.2
Molar Mass and Percent Composition
GPS & Essential Questions:
• Atoms & MolesSC2 (c):
Students will relate how the
Law of Conservation of Matter
is used to determine chemical
composition in compounds &
chemical reactions.
– (c): Apply concepts of the
mole and Avogadro’s
number to conceptualize
and calculate
• mass, moles and
molecules relationships
• molar volumes of gases
empirical/molecular
formulas
• What are the similarities of
atomic mass and molar mass?
• How is the molar mass
calculated for various
compounds?
• How do you convert between
moles, mass, and number of
atoms?
• How is the mass % of an
element in a compound
calculated?
Section 6.2
Molar Mass and Percent Composition
Average Atomic Masses Review:
• Because in the real world we use large amounts of
atoms and molecules, we use average masses in
calculations.
• Average mass is calculated from the isotopes of an
element weighted by their relative abundances.
Section 6.2
Molar Mass and Percent Composition
Average Atomic Masses of Elements
•
Most elements exist naturally as mixtures of isotopes.
•
Average atomic mass = the weighted average of the atomic masses of
the naturally occurring isotopes of an element.
•
Depends both on the mass and the relative abundance of each of the
element’s isotopes.
– Multiply the atomic mass of each isotope by its relative abundance
(expressed in decimal form) and adding the results.
– Ex: Cu
Section 6.2
Molar Mass and Percent Composition
A. Counting by Weighing
•
Objects do not need to have identical masses to be counted by weighing.
– All we need to know is the average mass of the objects.
•
To count the atoms in a sample of a given element by weighing we must
know the mass of the sample and the average mass for that element.
Section 6.2
Molar Mass and Percent Composition
A. Counting by Weighing
Averaging the Mass of Similar Objects
Example: What is the mass of 1000 jelly beans?
1. Not all jelly beans have the same mass.
2. Suppose we weigh 10 jelly beans and find:
3. Now we can find he average mass of a bean.
4. Finally we can multiply to find the mass of 1000
beans!
Section 6.2
Molar Mass and Percent Composition
A. Counting by Weighing
Averaging the Mass of Different Objects
 Two samples containing different types of
components (A and B), both contain the same
number of components if the ratio of the sample
masses is the same as the ratio of the masses of the
individual components.
Section 6.2
Molar Mass and Percent Composition
B. Atomic Masses: Counting Atoms by Weighing
•
Atoms have very tiny masses so scientists made a unit to avoid using very
small numbers.
1 atomic mass unit (amu) = 1.66 10-24 g
• The average atomic mass for an element is the weighted
average of the masses of all the isotopes of an element.
Section 6.2
Molar Mass and Percent Composition
Objectives
1. To learn to convert between moles and mass
2. To understand the definition of molar mass
3. To learn to calculate the mass percent of an element in
a compound
Section 6.2
Molar Mass and Percent Composition
Relating Mass to numbers of Atoms:
• Three very important concepts provide the basis for
relating masses in grams to numbers of atoms.
– mole
– Avogadro’s number
– molar mass
Section 6.2
Molar Mass and Percent Composition
Mole:
• SI unit for the amount of substance
• Abbreviated mol
• The amount of a substance that contains as many particles as
there are atoms in exactly 12 g. of carbon-12.
Section 6.2
Molar Mass and Percent Composition
Avogadro’s Number:
• Tells you the number of particles in exactly one mole of
pure substance (mole, particle, or molecules)
• Value means that exactly 12 grams of Carbon-12
contains 6.0221367 * 10 23 carbon 12 atoms.
• Value is
6.0221367 * 10 23 atoms, particles, or molecules
mole
• Allows you to convert between atoms and moles,
atoms and particles, or atoms and molecules.
• See EX:
Section 6.2
Molar Mass and Percent Composition
C. THE MOLE & AVOGADRO’S NUMBER
Section 6.2
Molar Mass and Percent Composition
Molar Mass:
• The molar mass of an
element is numerically
equally to the atomic mass
of the element in atomic
mass units.
•
• A molar mass of an element
contains one mole of atoms.
– EX: Helium-4 = 4.00 amu =
4.00 g/mol = 1 atom of
helium
Written in units of grams/mol
– Ex. Lithium
6.94 g/mol (molar mass)
6.94 amu (atomic mass units)
– EX: Lithium-6 = 6.94 amu =
6.94 g/mol = 1 atom of
helium
Section 6.2
Molar Mass and Percent Composition
A. Molar Mass
• A compound is a collection of atoms bound together.
• The molar mass of a compound is obtained by
summing the masses of the component atoms.
Section 6.2
Molar Mass and Percent Composition
A. Molar Mass
• For compounds containing ions the molar mass is
obtained by summing the masses of the component
ions.
Section 6.2
Molar Mass and Percent Composition
Calculations Using Molar Mass
• You can use molar mass (g/mol) as a conversion factor in
chemical calculations.
mass of the sample (g)
Moles of a compound =
molar mass of the compound (
g
)
mol
EX: How many moles are in 2.00 g of helium given the molar mass
of helium is 4.00 g/mol.
2.00 g * mole = 0.500 moles
4.00g
Section 6.2
Molar Mass and Percent Composition
Calculations Using Molar Mass
• Mass of a sample (g) = (moles of sample)(molar mass of compound)
EX: How many grams are in 2.00 mole of helium given the
molar mass of helium is 4.00 g/mol.
2.00 moles * 4.00g = 8.00 g
mole
Section 6.2
Molar Mass and Percent Composition
B. Percent Composition of Compounds
• Percent composition consists of the mass percent of
each element in a compound:
mass of a given element in 1 mol of compound
100%
Mass percent =
mass of 1 mol of compound