Transcript Section 3.2 Stoichiometry and Compound Formulas Compound Stoichiometry In this section… a. Element composition b.Percent composition from formulas c.

Section 3.2
Stoichiometry and Compound
Formulas
Compound Stoichiometry
In this section…
a. Element composition
b.Percent composition from formulas
c. Determining empirical formulas
d.Determining molecular formulas
e.Hydrated compounds
Element Composition:
You have a sample of a compound with known
mass.
What are the amounts of each element present?
Element Composition:
You have 2.50-mol sample of CH3CO2H.
What are the amounts of each element present?
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.
Determining Percent Composition:
Determining Percent Composition:
formula
*** If given just the formula, assume
you have 1 mol of the compound
What is the percent composition of each
element in N2O5?
%composition
Determining Empirical Formulas from Percent
Composition:
%composition
formula
*** If given just the percent compositions,
assume you have 100 g of the compound
Process:
1. for each element, mass present = % comp in g
2. convert g to mol for each element
3. compare mol of each element to get formula
Determining Empirical Formulas from
Percent Composition:
Use these % composition values
to determine the empirical formula:
Determining Empirical Formulas
from Percent Composition:
Some useful fraction-to-formula ratios:
1.2
1.25
1.33
1.4
1.5
1.6
1.67
1.75
1.8
1 + 1/5
1+¼
1 + 1/3
1 + 2/5
1+½
1 + 3/5
1 + 2/3
1+¾
1 + 4/5
6/5
5/4
4/3
7/5
3/2
8/5
5/3
7/4
9/5
A5B6
A4B5
A3B4
A5B7
A2B3
A5B8
A3B5
A4B7
A5B9
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 =
Determining Molecular Formulas from
Percent Composition and Molar mass
%composition
Process:
empirical formula
molecular formula
molar mass
1. for each element, mass present = % comp in g
2. convert g to mol for each element
3. compare mol of each element to
get empirical formula
4. compare empirical formula molar mass to
compound molar mass
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 =
Hydrated Compounds
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
final mass = mass anhydrous compound
moles H2O
= # of waters of
moles
hydration
anhydrous compound