Wed | Aug 29, 2007
 Chapter 3: Stoichiometry
– Focus on Sections 3.2 - 3.9
– Chemical Formulas
– Moles and Related Calculations
 Bring PRS units on Friday, Aug 31
 WebAssign HW due Thurs before midnight
Week 2
CHEM 1310 - Sections L and M
1
Mole
Avogadro’s Number, N0
The number of atoms in
exactly 12 grams of 12C.
6.022 x 1023 entities x mol-1
Mole (mol)
The amount of a substance
containing as many
elementary particles as
there are in 12 g of 12C.
Week 2
CHEM 1310 - Sections L and M
2
1
Counting in Chemistry
1 mole = different masses of elements
Week 2
CHEM 1310 - Sections L and M
3
Molar Mass
Given: Molecular Formula
Find: Molecular Weight (i.e. Molar Mass)
Sulfuric Acid:
H2 SO4
Solution:
(1) Multiply the atomic mass by the # of each atom type
(2) Sum the resulting weights
H (1.0079 g/mol) x 2 = 2.0158 g/mol
S (32.065 g/mol) x 1 = 32.065 g/mol
O (15.999 g/mol) x 4 = 63.996 g/mol
SUM: 98.077 g/mol
Week 2
CHEM 1310 - Sections L and M
4
2
Calculations
Given: Moles of an element or compound
Find: Weight of element or compound
EXAMPLE
What is the weight of 0.03 mol of gold?
Atomic Mass of Au: 196.97 g/mol
0.03 mol x 196.97 g = 6 g of Au
mol
Week 2
CHEM 1310 - Sections L and M
5
Calculations Involving Moles
Given: Weight of a molecule
Find: Number of atoms
EXAMPLE
How many atoms are contained in 1.5 g of NaOH?
FW of NaOH: 40 g/mol
1.5 g x
1 mol
40 g
x
6.022 x 1023 atoms
1 mol
= 2.3 x 1022 atoms
Week 2
CHEM 1310 - Sections L and M
6
3
Chemical Formulas
Empirical Formula: a chemical formula of a compound
that expresses the relative chemical amounts of its
elements in terms of the smallest integers.
Molecular Formula: a chemical formula that specifies
the actual number of atoms of each element in one
molecule.
Empirical Formula
CH2O
Week 2
Molecular Formula
C6H12O6
(Glucose)
CHEM 1310 - Sections L and M
7
Calculations with Formulas
Given: Mass Composition
Find: Empirical Formula
EXAMPLE
A sample of a tin and chlorine compound with a
mass of 2.57g was found to contain 1.17 g of tin.
What is the compound’s empirical formula
Atomic Mass of Tin: 118.710 g/mol
Atomic Mass of Chlorine: 35.453 g/mol
Week 2
CHEM 1310 - Sections L and M
8
4
Calculations with Formulas
Given: Empirical Formula and Molecular Mass
Find: Molecular Formula
EXAMPLE
The empirical formula of squalene is C3H5.
Its molar mass is 410.7 g/mol.
What is the molecular formula of squalene?
Week 2
CHEM 1310 - Sections L and M
9
More Calculations with Moles
Given: Amount of a compound
Find: Amount of an element in compound
Compute the mass (in grams) of
lithium in 65.4 g of Li2CO3
(lithium carbonate).
Week 2
CHEM 1310 - Sections L and M
10
5
More Complexity…
Given: Amount of an element within a compound
Find: Amount of a different element in compound
EXAMPLE
Chlorophyll, the green pigment in plants,
has the formula C55H72MgN4O5
If 0.0011g of Mg is available to a plant cell for
chlorophyll synthesis, how many grams of carbon
will be required to completely use up the
magnesium?
Week 2
CHEM 1310 - Sections L and M
11
Density & MW
Given: Density and Molecular Weight
Find: Molar Concentration (mol/L)
Density of NaCl: 2.16 g/cm3
FW of NaCl: 58.443 g/mol
cm3 = mL = 1 x 10-3 L
1 mol
58.443 g
x
2.16 g
mL
x 1000 mL
L
= 36.96 M NaCl
Week 2
CHEM 1310 - Sections L and M
12
6
Dilutions, Proportions
M1 x V1 = M2 x V2
What volume of 0.1M NaCl is needed to make
0.2 L of 10 mM NaCl?
(0.1M) x V1 = (10 x 10-3M) x (0.2L)
V1 = 0.02L or 20 mL
Remember this!
Week 2
CHEM 1310 - Sections L and M
13
On Friday
 Complete Chapter 3
– Chemical Equations
– How to balance chemical equations
– Calculations involving reactants and products
Week 2
CHEM 1310 - Sections L and M
14
7