Mass Relationships in Chemical Reactions

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Mass Relationships in Chemical Reactions
3.1-3.1 Atomic Mass Scale, Mole and Molar Mass
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Atomic Mass Scale
Pg. 1

• Atomic mass unit (amu) basic mass unit
• The atomic mass scale is a relative scale
• Basis of the atomic mass scale is the carbon-12
  isotope
• Mass of all other elements is measured relative
  to the mass of 1 carbon-12 atom
• 1 amu is defined as one-twelfth of the mass of 1
  carbon-12 atom

By definition 1 atom 12C weighs 12 amu
Atomic mass is the mass of an atom in atomic mass
units (amu)
On this scale 1H 1.008 amu 16O 16.00 amu
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Relationship of the units amu to g
Pg. 1
6.022 x 1023 amu 1 g
Example 1 6.022 x 1023 amu 1 g What is the
mass in grams of a 63Cu atom, which has a mass of
62.93958 amu?
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Average atomic mass
Pg. 1
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Average atomic mass
Pg. 1
Average atomic Mass the average mass of all of
the isotopes of an element, each one weighted by
its proportionate abundance
Example 2 Calculate the average atomic mass of
lithium given the following 6Li, exact mass
6.015121 amu, percent abundance 7.50 7Li,
exact mass 7.016003 amu, percent abundance
92.50
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Concept of the Mole
Pg. 2

• 1 carbon-12 atom weighs 12.00 amu
• 1 "mole" of carbon-12 atoms weighs exactly 12 g
• Mole (mol) the number of carbon-12 atoms in
  exactly 12 g of carbon-12.
• 1 "average" O atom weighs 16.00 amu
• 1 mole of O atoms weighs 16.00 g
• 1 "average" Ca atom weighs 40.08 amu
• 1 mole of Ca atoms weighs 40.08 g

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Avogadros Number
Pg. 2

• Experimentally, Avogadro determined that 1 mol
  contains 6.022 x 1023 atoms
• Vocabulary Word Number
• mole 6.022 x 1023
• dozen 12

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The mole (mol) is the amount of a substance that
contains as many elementary entities as there
are atoms in exactly 12.00 grams of 12C
1 mol NA 6.0221367 x 1023
Avogadros number (NA)
3.2
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eggs
shoes
Molar mass is the mass of 1 mole of
in grams
marbles
atoms
1 mole 12C atoms 6.022 x 1023 atoms 12.00 g 1
12C atom 12.00 amu
1 mole 12C atoms 12.00 g 12C 1 mole lithium
atoms 6.941 g of Li
For any element atomic mass (amu) molar mass
(grams)
3.2
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One Mole of
S
C
Hg
Cu
Fe
3.2
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Molar Mass of Atoms
Pg. 2

• Defined as the mass in grams of 1 mole of atoms
  (units g/mol)
• POINT OF EMPHASIS
• atomic mass in amu is numerically equal to molar
  mass in g/mol
• Atomic Mass Molar Mass
• C 12.01 amu 12.01 g/mol
• See Figure 3.2 Use a flowchart to help you
  convert from one quantity to another

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Conversions with Molar Mass and Avogadros Number
Pg. 2
Example 3 a. What is the atomic mass of Fe in
amu? b. What is the molar mass of Fe? c. How
many moles of Fe atoms are in 35.6 g? d. How
many atoms of Fe are in 35.6 g? e. What is the
mass of 1 atom of Fe in units of g?
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Molecular mass (or molecular weight) is the sum
of the atomic masses (in amu) in a molecule.
For any molecule molecular mass (amu) molar
mass (grams)
1 molecule SO2 64.07 amu 1 mole SO2 64.07 g
SO2
3.3
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Mass of Molecules and Ionic Compounds
Pg. 3

• i. Molecules
• The molecular mass of a molecule is simply the
  sum of the mass of each atom in the compound.
• Molecules
• Molecular Mass Molar Mass
• CO2 44.01 amu 44.01 g/mol
• ii. The formula mass of a formula unit of an
  ionic compounds is simply the sum of the mass of
  each atom in the formula.
• Ionic Compounds
• Formula Mass Molar Mass
• NaOH 40.00 amu 40.00 g/mol

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Example 4 Conversions with Molecules
Pg. 3

• Use a flowchart to help you convert from one
  quantity to another

Example 4 Aspartame, which is marketed as
Nutrasweet, has the molecular formula
C14H18N2O5. a) What is the molecular weight of
aspartame? b) What is the molar mass of
aspartame? c) How many molecules of aspartame
are in 10.0 g of aspartame? d) How many atoms of
nitrogen are in 10.0 g of aspartame?
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KE 1/2 x m x v2 v (2 x KE/m)1/2 F q x v x B
3.4
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Percent composition of an element in a compound
n is the number of moles of the element in 1 mole
of the compound
52.14 13.13 34.73 100.0
3.5
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Combust 11.5 g ethanol
Collect 22.0 g CO2 and 13.5 g H2O
6.0 g C 0.5 mol C
1.5 g H 1.5 mol H
g of O g of sample (g of C g of H)
4.0 g O 0.25 mol O
Empirical formula C0.5H1.5O0.25
Divide by smallest subscript (0.25)
Empirical formula C2H6O
3.6