1. Molecular formulas

2. Review of Moles Avogadros number Molar Mass
6.022 x Particles per mole of a substance
The particles can be atoms, molecules or ions
Molar Mass
The mass of one mole of a substance

3. Review Average Atomic Mass
weighted average of the atomic mass of an element’s isotopes
Found by:
taking into account the abundance of an atom in the environment

4. Formula Mass link
Formulas for covalent compounds show the elements and the number of atoms of each element in a molecule.
Formulas for ionic compounds show the simplest ratio of cations and anions in any pure sample.

5. Empirical vs actual vs molecular formulas
empirical formula
shows the simplest ratio for the relative numbers and kinds of atoms in a compound.
ammonium nitrate is NH2O
actual formula
shows the actual ratio of elements or ions in a single unit of a compound.
NH4NO2
molecular formula
is a whole-number multiple of the empirical formula

7. Determining Empirical Formulas
You can use the percentage composition for a compound to determine its empirical formula.
Convert the percentage of each element to g.
Convert from g to mol using the molar mass of each element as a conversion factor.
Compare these amounts in mol to find the simplest whole-number ratio among the elements.

8. Determining an Empirical Formula from Percentage Composition
Sample Problem G
Chemical analysis of a liquid shows that it is 60.0% C, 13.4% H, and 26.6% O by mass. Calculate the empirical formula of this substance.
link

9. Determining an Empirical Formula from Percentage Composition
Sample Problem G Solution
Assume that you have a g sample, and convert the percentages to grams.
for C: 60.0%  g = 60.0 g C
for H: 13.4%  g = 13.4 g H
for O: 26.6%  g = 26.6 g O

10. Determining an Empirical Formula from Percentage Composition
Sample Problem G Solution, continued
Convert the mass of each element into the amount in moles, using the molar mass.

11. The empirical formula is C3H8O.
Determining an Empirical Formula from Percentage Composition
Sample Problem G Solution, continued
The formula can be written as C5H13.3O1.66, but you divide by the smallest subscript to get whole numbers.
The empirical formula is C3H8O.