Transcript Molar Mass

Chapter 7
Chemical Quantities
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Atomic Mass
Atomic mass is the
• Mass of a single
atom in atomic
mass units (amu)
• Mass of an atom
compared to a 12C
atom
• Number below the
symbol of an
element
Periodic Table and Atomic Mass
Ag has
atomic
mass =
107.9 amu
S has atomic
mass =
32.07 amu
C has atomic mass
= 12.01 amu
Atomic Mass Factors
The atomic mass,
• Can be written as an equality
Example: 1 P atom = 30.97 amu
• Can be written as two conversion factors
Example:
1 P atom
and
30.97 amu
30.97 amu
1 P atom
Uses of Atomic Mass Factors
The atomic mass is used to convert:
• A specific number of atoms to mass (amu)
• An amount in amu to number of atoms
Learning Check
What is the mass in amu
of 75
silver atoms?
1) 107.9 amu
2) 8093 amu
3) 1.439 amu
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Learning Check
How many gold atoms
have a
mass of 1.85 x 105 amu?
1) 939 Au atoms
2) 3.64 x 107 Au atoms
3) 106 Au atoms
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Formula Mass
The formula mass is
• The mass in amu
of a compound
• The sum of the
atomic masses of
the elements in a
formula
Calculating Formula Mass
To calculate formula mass of Na2SO4,
• Multiply the atomic mass of each element by its subscript
• Total the masses
2 Na x 22.99 amu = 45.98 amu
1
1 S x 32.07 amu = 32.07 amu
1S
4 O x 16.00 amu = 64.00 amu
1O
Formula mass
Na2SO4
142.05 amu
Learning Check
Using the periodic table, calculate the
formula mass of aluminum sulfide Al2S3.
Collection Terms
A collection term states
a specific number of items.
• 1 dozen donuts
= 12 donuts
• 1 ream of paper
= 500 sheets
• 1 case = 24 cans
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
A Mole of Atoms
A mole (mol) is a collection that contains:
• The same number of particles as there are carbon
atoms in 12.01 g of carbon
• 6.022 x 1023 atoms of an element (Avogadro’s number).
1 Mol element
Number of Atoms
1 mol C
= 6.022 x 1023 C atoms
1 mol Na
= 6.022 x 1023 Na atoms
1 mol Au
= 6.022 x 1023 Au atoms
A Mole of a Compound
A mole
• Of a covalent compound has Avogadro’s number of
molecules
1 mol CO2 = 6.022 x 1023 CO2 molecules
1 mol H2O = 6.022 x 1023 H2O molecules
• Of an ionic compound contains Avogadro’s number of
formula units
1 mol NaCl
= 6.022 x 1023 NaCl formula units
1 mol K2SO4 = 6.022 x 1023 K2SO4 formula units
Samples of One-Mole Quantities
Avogadro’s Number
Avogadro’s number 6.022 x 1023 can be written as an
equality and two conversion factors.
Equality:
1 mol
= 6.022 x 1023 particles
Conversion Factors:
6.022 x 1023 particles
1 mol
and
1 mol
6.022 x 1023 particles
Using Avogadro’s Number
Avogadro’s number is used to convert
Moles of a substance to Particles.
How many Cu atoms are in 0.50 mol Cu?
0.50 mol Cu x 6.022 x 1023 Cu atoms
1 mol Cu
= 3.0 x 1023 Cu atoms
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Using Avogadro’s Number
Avogadro’s number is used to convert
Particles of a substance to Moles.
How many moles of CO2 are in
2.50 x 1024 CO2 molecules?
2.50 x 1024 CO2 x
1 mol CO2
6.022 x 1023 CO2
= 4.15 mol CO2
Learning Check
1. The number of atoms in 2.0 mol Al is
A. 2.0 Al atoms
B. 3.0 x 1023 Al atoms
C. 1.2 x 1024 Al atoms
2. The number of moles of S in 1.8 x 1024 atoms S is
A. 1.0 mol S atoms
B. 3.0 mol S atoms
C. 1.1 x 1048 mol S atoms
Subscripts and Moles
The subscripts in a formula state
• The relationship of atoms in the formula.
• The moles of each element in 1 mol of compound.
Glucose
C6H12O6
In 1 molecule: 6 atoms C 12 atoms H 6 atoms O
In 1 mol:
6 mol C
12 mol H
6 mol O
Subscripts State Atoms and
Moles
The subscripts are used to write conversion factors for
moles of each element in 1 mol compound. For aspirin
C9H8O4, the following factors can be written:
9 mol C
1 mol C9H8O4
8 mol H
1 mol C9H8O4
4 mol O
1 mol C9H8O4
and
1 mol C9H8O4
9 mol C
1 mol C9H8O4
8 mol H
1 mol C9H8O4
4 mol O
Learning Check
How many O atoms are in 0.150 mol aspirin
C9H8O4?
Molar Mass
The molar mass
• Is the mass of one mole
of an element or
compound
• Is the atomic mass
expressed in grams
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Molar Mass from Periodic Table
Molar mass
• Is the atomic
mass
expressed in
grams
Molar Mass of a Compound
The molar mass of a compound is the sum of the
molar masses of the elements in the formula.
Example: Calculate the molar mass of CaCl2.
Element
Ca
Number Atomic Mass Total Mass
of
Moles
1
40.08 g/mol
40.08 g
Cl
2
CaCl2
35.45 g/mol
70.90 g
110.98 g
Some One-Mole Quantities
32.07 g
55.85 g
58.44 g
294.20 g
342.30 g
Learning Check
Prozac, C17H18F3NO, is an antidepressant that inhibits
the uptake of serotonin by the brain. What is the molar
mass of Prozac?
1) 40.06 g/mol
2) 262.0 g/mol
3) 309.3 g/mol
Molar Mass Factors
Molar mass conversion factors:
• Are written from molar mass
• Relate grams and moles of an element or compound.
Example: Write molar mass factors for methane CH4 used
in gas cook tops and gas heaters.
Molar mass:
1 mol CH4 = 16.04 g
Conversion factors:
16.04 g CH4
1 mol CH4
and
1 mol CH4
16.04 g CH4
Learning Check
Acetic acid C2H4O2 gives the sour taste to vinegar.
Write two molar mass conversion factors for acetic
acid.
Calculations Using Molar Mass
Molar mass factors are used to convert between the
grams of a substance and the number of moles.
Grams
Molar mass factor
Moles
Moles to Grams
Aluminum is often used to build lightweight bicycle
frames. How many grams of Al are in 3.00 mol Al?
Molar mass equality: 1 mol Al = 26.98 g Al
Setup with molar mass as a factor:
3.00 mol Al x
26.98 g Al
= 80.9 g Al
1 mol Al
molar mass factor
for Al
Learning Check
Allyl sulfide C6H10S is a
compound that has the odor
of garlic. How many moles of
C6H10S are in 225 g?
Grams, Moles, and Particles
A molar mass factor and Avogadro’s number convert:
• Grams to particles
molar mass
(g
Avogadro’s number
mol
particles)
• Particles to grams
Avogadro’s number
(particles
molar mass
mol
g)
Learning Check
How many H2O molecules are in 24.0 g
H2O?
1) 4.52 x 1023
2) 1.44 x 1025
3) 8.02 x 1023
Percent Composition
Percent composition
• Is the percent by mass of each element in a formula
Example: Calculate the percent composition of CO2.
CO2 = 1 C(12.01g) + 2 O(16.00 g) = 44.01 g/mol)
12.01 g C
44.01 g CO2
x 100
=
27.29 % C
32.00 g O
x 100
44.01 g CO2
=
72.71 % O
100.00 %
Learning Check
The chemical isoamyl
acetate C7H14O2 gives the
odor of pears. What is the
percent carbon in isoamyl
acetate?
1) 7.102% C
2) 35.51% C
3) 64.58% C
Types of Formulas
The molecular formula
• Is the true or actual number of the atoms in a molecule
The empirical formula
• Is the simplest whole number ratio of the atoms
• Is calculated by dividing the subscripts in the molecular
formula by a whole number to give the lowest ratio
C5H10O5  5 = C1H2O1 =
molecular
formula
CH2O
empirical formula
Some Molecular and Empirical
Formulas
• The molecular formula is the same or a
multiple of the empirical.
Learning Check
A. What is the empirical formula for C4H8?
1) C2H4
2) CH2
3) CH
B. What is the empirical formula for C8H14?
1) C4H7
2) C6H12
3) C8H14
C. Which is a possible molecular formula for CH2O?
1) C4H4O4
2) C2H4O2
3) C3H6O3
Learning Check
A compound contains 7.31 g Ni and
20.0 g Br.
• Calculate its empirical (simplest)
formula.
• If the compound has a molar mass of
437 g/mol, calculate the molecular
formula.
Learning Check
Aspirin is 60.0% C, 4.5% H and 35.5%
O. Calculate its empirical (simplest)
formula.