The Mole

Atomic Mass

• Relative weight - the atomic mass unit (amu) is used to describe the mass of an atom relative to a carbon-12 isotope.

• 1mole of any element makes the relative weight equal to the gram weight.Element relative weight atomic

weightH 1 amu 1 g/molO 16 amu 16 g/mol

The Mole

• A mole is a quantity - 1 mole of any substance contains 6.02 x 1023 particles– Example: – 1 dozen = 12 things– 1 pair = 2 things– 1 ream = 500 things

• Avogadros’s number = 6.02x1023

Molar Mass (MM)• The mass in grams of 1 mole of a

molecule is the molar mass.

• The mass in grams of an ionic compound is called the formula mass.

• A formula unit is an ionic compound.

Molar or Formula Mass Calculations

Example: 1. What is the formula mass of CuO?

Cu = 63.55 g/mol O = 16.00 g/molCuO = 63.55 + 16.00 = 79.55 g/mol

2. What is the molar mass of H2O?H = 1.01 g/mol, O = 16.00 g/mol

H2O = (1.01 x 2) + 16.00 = 18.02 g/mol

Molar Mass• Formula mass and molar mass are

calculated the same way. Formula mass refers to ionic compounds and molar mass refers to molecules.

• The formula gives the mole ratio of each atom in the compound.

Example: 1 mole of H2O = 2 mole of hydrogen atoms and 1 mole of oxygen atoms

Mole Conversions

• Balanced chemical equations give the mole ratio of reactants and products.– 2H2 + O2 2H2O : 2 moles of hydrogen

reacts with 1 mole of oxygen to make 2 moles of water.

• To find grams or molecules/formula units you must convert using moles.

Converting Between Moles and grams

1. How many grams is 2 moles of water? Use the molar mass to convert between

moles and grams:

2 moles H2O x 18.02 g = 36.04 g mole

2. How many moles are in 5.0 grams of H2O?

5 g H2O x mole = .28 moles18.02 g

Converting between Molecules and Moles

1. How many molecules are in 2 moles of water?

Use Avogadro's number to convert from moles to molecules.

2 moles H2O x 6.02 x 1023 molecules =

mole

1.20 x 1024 molecules