The Mole: A Shortcut for Chemists
Source: http://en.wikipedia.org/wiki/Mole_(animal)
S-C-8-1_The Mole Presentation
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 amu
For 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
# of moles
Formula weight
Mass (in grams)
(grams):
×
=
1 mol
• If you want to convert from mass (grams) to amount
1 mol
# of moles
(moles): mass (grams) ×
=
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