Section 3.2 Stoichiometry and Compound Formulas

1
Section 3.2 Stoichiometry and Compound Formulas
2
Compound Stoichiometry

• In this section
• Element composition
• Percent composition from formulas
• Determining empirical formulas
• Determining molecular formulas
• Hydrated compounds

3
Element Composition
You have a sample of a compound with known mass.
What are the amounts of each element present?
4
Element Composition
You have 2.50-mol sample of CH3CO2H. What are
the amounts of each element present?
5
Percent Composition
A compound is formed from two or more elements in
a set ratio of atoms. The ratio of atoms gives
the chemical formula. The ratio of masses of
those atoms gives the percent composition.
6
Determining Percent Composition
7
Determining Percent Composition
formula composition
If given just the formula, assume
you have 1 mol of the compound
What is the percent composition of each element
in N2O5?
8
Determining Empirical Formulas from Percent
Composition
composition formula
If given just the percent compositions,
assume you have 100 g of the compound

• Process
• for each element, mass present comp in g
• convert g to mol for each element
• compare mol of each element to get formula

9
Determining Empirical Formulas from Percent
Composition
Use these composition values to determine the
empirical formula
10
Determining Empirical Formulas from Percent
Composition
Some useful fraction-to-formula ratios 1.2
1 1/5 6/5 A5B6 1.25 1 ¼
5/4 A4B5 1.33 1 1/3
4/3 A3B4 1.4 1 2/5
7/5 A5B7 1.5 1 ½
3/2 A2B3 1.6 1 3/5
8/5 A5B8 1.67 1 2/3
5/3 A3B5 1.75 1 ¾
7/4 A4B7 1.8 1 4/5 9/5 A5B9
11
Empirical vs. Molecular Formulas
Empirical Formula Molecular Formula
If you know the empirical formula, how do you
find the molecular formula?
answer molar masses of the two
Example Molar mass of empirical formula (CH2)
14.03 g/mol Molar mass of compound measured by
experiment to be 56.11.
Molecular Formula
12
Determining Molecular Formulas from Percent
Composition and Molar mass
composition empirical formula
molecular formula
molar mass

• Process
• for each element, mass present comp in g
• convert g to mol for each element
• compare mol of each element to
• get empirical formula
• compare empirical formula molar mass to
• compound molar mass

13
Determining Molecular Formula
A compound has molar mass of 110.11 g/mol and
contains 65.45 C 5.493 H 29.06 O
Empirical Formula
Empirical Formula (C3H3O)molar mass 55.06 g/mol
Molecular formula
14
Hydrated Compounds
15
Hydrated Compounds
How do you determine the number of waters of
hydration?
Answer heat off the water and see how much mass
decreases
mass loss mass H2O
moles H2O


of waters of final mass
mass anhydrous compound
moles hydration

anhydrous compound