Chemical Quantities
Chapter 7
2 lb bag of sugar
STOP
5 lb bag of potatoes 2 liters of soda-pop
3 gross of M&M’s
10 gallons of gasoline
1 dozen Krispy Kream doughnuts
•We describe quantities by weighing them, how
much space they take up, or by counting them.
•We do the same in chemistry
2 g of NaCl
(weight)
3 mL of H20
(volume)
5 moles of MgI
(counting)
• A mole is a quantity of particles, just as…
1 dozen = 12 things
1 mole = 6.02 x 1023 particles
• “Particles” usually measured in moles are
atoms, molecules, ions, and formula units
Important # to remember…
Avogrado’s Number
1 mole =
23
6.02 x 10
Examples: moles  atoms
How many atoms of Al are in 1.50 mol of Al?
Conversion:
1 mole = 6.02 x 1023 atoms
1.50 mol of Al
6.02 x 1023 atoms
1 mole
=
9.03 x 1023 atoms
of Al
Examples: moles  molecules  atoms
How many atoms of H are there in 3.0 moles
of H2O?
Conversions:
1 mole = 6.02 x 1023 molecules
H2O molecule = 2 atoms of Hydrogen
3 moles of H2O
6.02 x 1023 molec.
1 mole
= 3.6 x 1024 atoms H
2 atoms H
1 H2O molecule
Question 1:
•
Which contains more molecules:
–
–
–
A.
B.
C.
D.
1.00 mol H2O2
1.00 mol C2H6
1.00 mol CO
H2O2
C2H6
CO
All contains 6.02 x 1023 molecules
Question 2:
•
Which contains more atoms:
–
–
–
A.
B.
C.
D.
1.00 mol H2O2
1.00 mol C2H6
1.00 mol CO
H2O2
C2H6
CO
All contains 6.02 x 1023 atoms
Question 3:
• Determine the number of atoms
in 3.00 mol Sn
A. 1.81 x 1025 atoms Sn
B. 1.81 x 1024 atoms Sn
C. 4.98 x 10-24 atoms Sn
D. None of the above
Molar Mass
• Determined simply by looking at the
periodic chart
• Molar mass = Atomic Mass
20
Ca
40.08
* Thus,
1 mol Ca = 40 g
Molar Mass
Question 4
• Calculate the mass of 1.00 mol of
copper
A. 29 g Cu
B. 63 g Cu
C. 64 g Cu
D. 1 g Cu
Examples: grams  moles
Calculate the number of moles in 367 g of silver?
Conversion:
1 mole Ag = 108 g
367 g Ag
1 mol Ag
108 g Ag
=
3.40 mol Ag
Molar Mass (compound)
• Mass of 1 mole in a compound
• Determined by adding the molar mass of
each atom in that compound
What is the gram molecular mass of H2O?
2 atoms H
= 1gx2
1 atom O
= 16 g x 1
= 18 g
Thus… 1 mole of H2O = 18 g
Question 5:
• Calculate the molar mass of
diatomic nitrogen
A. 7 g N
B. 14 g N2
C. 28 g N2
D. 14 g N
Examples: moles  grams
What is the mass of 3.40 moles of H2O?
Conversion:
1 mole H2O = 18 g
3.4 mol H2O
18 g
1 mole H2O
=
61.2 g H2O
Examples: moles  grams
What is the mass of 5.60 moles of ammonium
carbonate?
Conversion:
ammonium carbonate - (NH4)2CO3
N: 2 x 14 g =
28 g
H: 8 x 1 g
=
8g
C: 1 x 12 g
= 12 g
O: 3 x 16 g =
48 g
1 mol (NH4)2CO3 = 96 g
5.60 mol (NH4)2CO3
96 g
1 mol (NH4)2CO3
=
538 g
Examples: moles  atoms
How many atoms is this?
Conversions:
1 mole = 6.02 x 1023 molecules
1 (NH4)2CO3 molecule = 14 atoms
5.6 moles (NH4)2CO3 6.02 x 1023 molecules
1 mole (NH4)2CO3
= 4.72 x 1025 atoms
14 atoms
1 molecule (NH4)2CO3
• When dealing with gasses,
volume is determined by using
the conversion:
1 mole = 22.4 L
Examples: moles  volume
How much volume is 5.60 moles of CO2?
Conversion:
1 mole = 22.4 L
5.6 mol CO2
22.4 L
1 mol
=
125 L CO2
STOP
Review of conversions for moles
1 mole = 6.02 x
23
10
particles
1 mole = _____ grams
1 mole = 22.4 L
Examples:
How many grams of Al are in 2.0 mol Al?
Conversion:
1 mole Al = 27 g
2.0 mol Al
27 g Al
1 mol Al
=
54 g Al
Examples:
How many molecules are in 5 mol of N2O3?
Conversion:
1 mole N2O3 = 6.02 x 1023 molecules
6.02 x 1023 molecules
5 mol N2O3
1 mol N2O3
=
3.01 x 1024 molecules N2O3
Examples:
What is the mass of 8 L of CO2? (Hint: 2
step conversion)
Conversion:
1 mol = 22.4 L
1 mol CO2 = 44 g
8 L CO2
1 mol
22.4 L
44 g CO2
1 mol CO2
= 15.7 g CO2
Percent Composition:
• Describes the relative amounts of each
element in a compound (% by mass)
% mass of =
element
Grams of element in compound
Grams of compound
x 100
What is the % mass of each element when you combine
9.41g of Ca with 5.99g of S ?
Grams of Compound = 9.41g + 5.99g = 15.40g
% Ca =
9.41 x 100
15.40
= 61%
%S=
5.99
x 100
15.40
= 39%
If you know the chemical formula, you
can calculate % composition using
molar mass
Ex.) Calculate the % comp. of ethane (C2H6)
C2 =
24 g
H6 =
6g
C2H6 =
30 g
%C=
24
30
% C = 80%
x 100
%H=
6
30
% H = 20%
x 100
% comp. can be used to calculate the # of
grams of an element in a specific amount of a
compound
Ex.) Calculate the mass of carbon in 30 g of
ethane (C2H6)
From prev. example: %C in C2H6 = 80% of C
30 g C2H6
x
.80
(80 %) C
=
24 g C
Empirical Formula
• The lowest whole-number ratio of the elements
in a compound
Ie.) The E.F. of hydrogen peroxide (H2O2) is HO
• The E.F. can be determined by % comp.
Ex.) Calculate the formula for a compound
that is 67.6% Hg, 10.8% S, & 21.6% O.
• Assume you have 100 g of this compound
 67.6 g Hg
 10.8 g S
 21.6 g O
• Determine the # of moles of each
67.6 g Hg
1 mole Hg
200 g Hg
10.8 g S
1 mole S
32 g S
21.6 g O
1 mole O
16 g O
= .337 mol Hg
= .337 mol S
= 1.35 mol O
But the subscripts need to be whole #’s
• Find the smallest subscript and divide
each by it
.337 mol Hg
.337 mol
=
1 mol Hg
.337 mol S
.337 mol
=
1 mol S
1.35 mol O
.337 mol
=
4 mol O
HgSO4
Empirical
Formula