S-C-8-1_The Mole Presentation

The Mole: A Shortcut for Chemists

Source: http://en.wikipedia.org/wiki/Mole_(animal)

The Mole

• The mole is a counting unit for chemists, the same way a baker uses a dozen.

• 1 dozen = 12 objects• 1 mole = 6.02 × 1023 objects =

602,000,000,000,000,000,000,000 objects• That’s almost a trillion trillion!• 6.02 × 1023 is called Avogadro’s number.• “Mole” in writing; “mol” in calculations.

Representative Particles

• Moles can be used to count “representative particles:” atoms, molecules, ions, and formula units.

• The representative particle of an ionic compound is the formula unit.

• The representative particle of a covalent compound is the molecule.

• The representative particle of an element is the atom.

Atomic Masses

• What do the atomic masses on the periodic table represent?

• Carbon has an atomic mass of 12; this means a carbon atom weighs 12 atomic mass units (amu).

• The actual mass of an atom of carbon is only 2 x 10-23 grams.

• Practice: What is the atomic mass of one molecule of CO2?

12.01 + 16.00 + 16.00 = 44.01

Formula Weight

• Formula weight: The weight of a molecule or an ionic compound.

• Molecule: CO2

12.01 + 16.00 + 16.00 = 44.01 amuFor molecules, formula weight is also

called the “molecular weight.”• Ionic compound: NaCl 22.99 + 35.45 = 58.44 amu

Molar Mass

• Molar mass = the mass of one mole.• For atoms, molar mass is the same as atomic mass.• The molar mass of a compound allows you to convert

between the amount of the element (moles) and its mass (grams).

• If you want to convert from amount (moles) to mass (grams):

• If you want to convert from mass (grams) to amount (moles):

 # of moles×

 Formula weight=

Mass (in grams)  1 mol  

mass (grams)×

1 mol=

# of moles  formula weight    

Molar Mass Examples

• 1 mol N = 14.01 g N• Written as 14.01 g/mol• Calculate the following molar masses:

•Br•CaF2

•NO2

•NaCl

Molar Mass Examples: Solutions

Calculate the following molar masses:• Br

79.90• CaF2

40.08 + 19.00(2) = 78.08 g/mol• NO2

14.01 + 16.00(2) = 46.01 g/mol• NaCl

22.99 + 35.45 = 58.44 g/mol

Conversion Factors: Mole-Mass Problems

• The red portions in the examples above are the conversion factors.

• Conversion factor: A ratio equal to one that expresses the same quality in two different ways.

• Another example: To find the number of eggs in 3 dozen:3 dozen x 12 eggs/dozen = # of eggs

Mole-Mass Problems

1. What is the mass of 0.50 mol HCl?

2. What is the mass of 2 mol Zn?

3. How many moles are in 1.5 g of Cu?

4. How many moles are in 50 g H2SO4?

Mole-Mass Answers

1. What is the mass of 0.50 mol HCl?0.50 mol × 36.46 g/mol = 18.23 g HCl

2. What is the mass of 2 mol Zn?2 mol × 65.41 g/mol = 130.82 g

3. How many moles are in 1.5 g of Cu?1.5 g × 1 mol/63.55 g = 0.02 mol

4. How many moles are in 50 g H2SO4?

50 g × 1 mol/98.09 g = 0.51 mol