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:
/5 6/ A5B6
¼ /4 A4B5
/ /3 A3B4
/ /5 A5B7
½ /2 A2B3
/ /5 A5B8
/ /3 A3B5
¾ /4 A4B7
/ /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) = g/mol
Molar mass of compound measured by experiment to be
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 g/mol and contains:
65.45% C
5.493% H
29.06% O
Empirical Formula =
Empirical Formula (C3H3O)molar mass = 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