1. Counting units such as dozen or gross are commonly used to deal with large quantities of items. No matter what we are counting, a dozen means 12, whether we are counting oranges, pencils, or even elephants.

2. Review in your text – a mole
Pages 83 – 87 definitions and practice
Avogadro’s number, rounded to 6.02 x 10 23
is used to describe the amount of a substance so that chemists can determine the molar mass in one mole. Using the average atomic mass for each atom we can use dimensional analysis to determine the mass of 1 mole in grams for any substance .
CHECK FOR UNDERSTANDING : COMPLETE PRACTICE PROBLEMS PG 85

3. What is a mole?
The mole is a counting unit used by chemists to express the number of atoms or molecules in a sample.
1 mole = 6.02 x1023 “things”
1 mole of oranges contains 6.02 x1023 oranges.
1 mole of helium gas contains 6.02 x1023 helium atoms.
1 mole of water contains 6.02 x1023 water molecules.
1 mole of sodium chloride contains 6.02 x1023 sodium chloride formula units.

4. Molecules vs. Formula Units
The simplest unit of a molecular (covalent) compound is a molecule.
Ionic compounds contain large numbers of ions held together in a lattice structure, they do not exist as single molecules. Instead, a formula unit represents the simplest ratio of atoms in an ionic compound.

5. Counting Atoms and Ions
Substance
Formula
Number and Kind of
Particles in one Mole
Atomic Nitrogen N
6.02 x1023 N atoms
Molecular Nitrogen N2
6.02 x1023 N2 molecules
_______________ N atoms
Nitrogen Dioxide
NO2
6.02 x1023 NO2 molecules
_______________ atoms
Sodium Chloride
NaCl
6.02 x1023 NaCl formula units
_______________ ions
Sodium Nitride
Na3N
6.02 x1023 Na3N formula units
_______________ Na+ ions
_______________ N3– ions

6. Formula Weight
Formula Weight – the sum of the atomic weights of the atoms in a chemical formula.
Represents the mass of one molecule
Measured in atomic mass units (amu)
Calculate the formula weight of each of the following compounds: O2
NaCl
Ca3(PO4)2

7. Molar Mass Molar Mass – the mass in grams of one mole of a substance.
By definition a mole of 12C has a mass of 12 grams.
Because of this, the mass of a single atom of an element (in amu) is numerically equal to the mass (in grams) of 1 mole of that element.
1 Mg atom = 24.3 amu
1 mole of Mg atoms = 24.3 g
1 H2O molecule = 18.0 amu
1 mole of H2O molecules= 18.0 g
1 O2 molecule = 32.0 amu
1 mole of O2 molecules = 32.0 g

8. Why do we need the mole?
Because atoms are extremely small and we usually deal with large amounts of them. Think about how many atoms are in 1 mole:
602,000,000,000,000,000,000,000
The mole is a useful tool for chemists because it provides a relationship between the number of atoms or molecules in a substance (which we cannot measure easily) and their mass (which we can measure easily).

9. Mole Conversions 1 mole = molar mass in grams
(molar mass can be found using the periodic table).
1 mole = 6.02 x1023 particles
(particles can be atoms, molecules, or formula units)
1 mole = Liters of any gas at STP
(STP represents standard temperature and pressure conditions of 0 °C and 1 atmosphere respectively).