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.

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

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.

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.

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

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

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

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).

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 = 22.41 Liters of any gas at STP (STP represents standard temperature and pressure conditions of 0 °C and 1 atmosphere respectively).