Section 6.2
Molar Mass and Percent Composition
Objectives
1. To understand the definition of molar mass
2. To learn to convert between moles and mass
3. To learn to calculate the mass percent of an element in
a compound
Section 6.2
Molar Mass and Percent Composition
A. Molar Mass
• A compound is a collection of atoms bound together.
1 mole of
methane
1 mole of
carbon
and 4
moles of
hydrogen
• The molar mass of (“one mole of”) a compound is
obtained by summing the masses of the appropriate
number of moles of 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. This is sometimes called the formula weight.
Section 6.2
Molar Mass and Percent Composition
What is the Molar Mass of the following Compounds?
Na3N
CS2
NH4Br
C2H5OH
H2SO3
H2SO4
MgSO4
Section 6.2
Molar Mass and Percent Composition
A. Molar Mass
Calculations Using Molar Mass
• Moles of a compound (g) =
mass of the sample (g)
molar mass of the compound (
g
)
mol
• Mass of a sample (g) = moles of sample (mol) x
molar mass of compound (g/mol)
• Number of atoms in moles of a sample = number of moles x
number of atoms in formula x Avogadro’s number
Section 6.2
Molar Mass and Percent Composition
Relationships Between Moles, Mass and Atoms
Avogadro’s
Number
6.022x1023
Atoms
Atomic Mass
from Periodic
Table
Moles
Mass
Section 6.2
Molar Mass and Percent Composition
Relationships Between Moles, Mass, Molecules and
Atoms
Avogadro’s
Number
6.022x1023
Molecules
Molecular
Formula
Atoms
Molar Mass
using Periodic
Table
Moles
Mass
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 percent =
mass of a given element in 1 mol of compound
100%
mass of 1 mol of compound
What are the mass percentages of C, H and O in Ethanol?
Section 6.2
Molar Mass and Percent Composition
Calculate the Mass Percent of These Compounds
LiBr
CuOH
SrSO4
C2H4O
C6H12O6