1. Percent Composition, Empirical Formulas, and Molecular Formulas

2. Percent Composition
percent composition: the percentage by mass of each element in a compound
Part
_______
Percent =
x 100%
Whole
% element =
mass of element in compound
__________________________
mass of compound
x 100%

3. Percent Composition Example
% element =
mass of element in compound
__________________________
mass of compound
x 100%
What is the percent composition of the element carbon in CH3COOH?

4. Percent Composition Example
% element =
mass of element in compound
__________________________
mass of compound
x 100%
What is the percent composition of each element in calcium chloride?

5. Empirical Formulas
Molecular Formula: chemical formulas that indicate the actual numbers and types of atoms in a molecule
Empirical Formula: chemical formulas that give only the relative number of atoms
the “simplified/reduced form” of the formula
*Often the same!

6. Empirical Formula Example
Remember: A chemical formula tells us the number of MOLES of each element – you MUST convert grams to moles!!!
Example:
A compound contains g Fe and g O. What is its empirical formula?

7. Empirical Formula Example
Remember: A chemical formula tells us the number of MOLES of each element – you MUST convert grams to moles!!!
Example:
What is the empirical formula for a compound with 18.8% nickel and 81.2% iodine?

8. Hydrates
Hydrate: a chemical compound that contains water as part of its structure

9. Hydrate Example
You perform an experiment to determine the molecular formula of a nickel sulfate hydrate, NiSO4•xH2O. The mass of the hydrate before heating is 3.00 g. The mass of the anhydrous compound after heating is 1.76 g. What is the molecular formula of the hydrate?