CHAPTER 10 CHAPTER 10 THE MOLETHE MOLE

The mole is a number (6.02 x 10The mole is a number (6.02 x 102323))

It is a term like the term “dozen”It is a term like the term “dozen”

It was chosen by chemists to make It was chosen by chemists to make working with atomic weights easier.working with atomic weights easier.

It is the number of atoms in one It is the number of atoms in one atomic weight (in grams) of an element.atomic weight (in grams) of an element.

10.81 grams of boron contains one 10.81 grams of boron contains one mole of boron atoms.mole of boron atoms.

1.0 gram of H contains one mole of H 1.0 gram of H contains one mole of H atoms.atoms.

Molecular weightsMolecular weights

When using atomic weights we can When using atomic weights we can choose the units (AMU/atom or choose the units (AMU/atom or grams/mole)grams/mole)

18 grams of H18 grams of H22O contains one mole O contains one mole

of water molecules.of water molecules.

Molecular weight of HMolecular weight of H22O = grams/mole O = grams/mole

1g +1g + 1g +1g + 16g 16g

H +H + H +H + OO

What is the molecular weight What is the molecular weight of ethanol (Cof ethanol (C22HH66O)?O)?

2 carbons2 carbons 12 + 1212 + 12 2424

6 H6 H 6 (1)6 (1) 66

1 oxygen1 oxygen 1616 1616

----------

molecular weight of ethanol =molecular weight of ethanol = 46 46 g/moleg/mole

oror 46 46 AMU/molecule AMU/molecule

empirical formula empirical formula

It is the formula of a substance in It is the formula of a substance in terms of the lowest combining ratio terms of the lowest combining ratio of the elements.of the elements.

Hydrogen peroxide molecular Hydrogen peroxide molecular formula is Hformula is H22OO22

The empirical formula is HO The empirical formula is HO

Formulas of ionic compounds are always Formulas of ionic compounds are always empiricalempirical

Experimentally determining an Experimentally determining an empirical formulaempirical formula

Determine the mass of each element Determine the mass of each element in a sample of the compound.in a sample of the compound.

From the mass determine the moles of From the mass determine the moles of each element.each element.

Use the mole ratio of the elements to Use the mole ratio of the elements to determine the empirical formula.determine the empirical formula.

Example: Example:

What is the empirical formula of a liquid What is the empirical formula of a liquid if it was found to contain 1.2 grams of if it was found to contain 1.2 grams of hydrogen and 19 grams of oxygen?hydrogen and 19 grams of oxygen?

Avogadro’s relationshipAvogadro’s relationship

Equal volumes of any gas at the Equal volumes of any gas at the same temperature and pressure same temperature and pressure will have the same number of will have the same number of molecules.molecules.

One mole of gas at standard One mole of gas at standard temperature and pressure (25temperature and pressure (25ooC, C, 1 atm) will have a volume of 22.4 L1 atm) will have a volume of 22.4 L