Unit 5: The Mole
Molar Mass
• The sum of the atomic masses of all the
atoms in the formula of a compound or
molecule
Calculating Molar Mass
• Calculate the molar mass for H2O:
– H = 2 x 1.0079 g = 2.0158 g
– O = 1 x 15.999 g = 15.999 g
Molar mass of water = 18.0148 g
• Calculate the molar mass for Fe2(SO4)3:
– Fe = 2 x 58.847 g
= 117.694 g
– S = 3 x 32.06 g
= 96.18 g
– O = 12 x 15.999 g
= 191.988 g
Molar mass of iron (III) sulfate = 405.862 g
Practice Practice Practice!!!
The Mole
• A counting word, just like a dozen
– A dozen = 12
– A mole = 6.02 x 1023
• Two moles of marbles = 2 x 6.02 x 1023
• Three moles of hair pins = 3 x 6.02 x 1023
• 6.02 x1023 also known as Avogadro’s #
– Amedeo Avogadro (Italian Scientist) aided
in this development
Mole Conversion Factor
Molar mass (g) of substance =
6.02 x 1023 molecules of substance =
1 mole of substance
Mole Conversion Factor
Possibilities
• 1 mole
molar mass (g)
• molar mass (g)
1 mole
• 1 mole
6.02 x 1023 atoms or molecules
• 6.02 x 1023 atoms or molecules
1 mole
Mole Conversion Factor
Possibilities
• molar mass (g)
6.02 x 1023 atoms or molecules
• 6.02 x 1023 atoms or molecules
molar mass (g)
Use the word atoms when describing a single
element; use the word molecule when
describing molecules and compounds
Mole Conversion Examples
Molar mass
= Avogadro’s #
= 1 mole
18.0148 g of H2O = 6.02 x 1023 molecules of H2O = 1 mole of H2O
405.862 g of Fe2(SO4)3 = 6.02 x 1023 of Fe2(SO4)3 = 1 mole of Fe2(SO4)3
6.941 g of Li = 6.023 x 1023 atoms of Li = 1 mole of Li
Mole Conversion Practice
Problem 1
• If 5,000,000 chlorine atoms are used to
disinfect water, how many moles is
that?
5,000,000 atoms Cl x
1 mole Cl
= 8.3 x 1023 mole Cl
6.02 x 1023 atoms Cl
8.3 x 1023 mole Cl  8 x 1023 mole Cl
Correct Sig Fig
Mole Conversion Practice
Problem 2
• Thirty moles of carbon monoxide could
be a lethal amount. How many grams is
that?
30 moles CO x 28.01 g of CO = 840.3 g of CO
1 mole CO
840.3 g CO  800 g CO
Correct Sig Fig
Mole Conversion Practice
Problem 3
• One liter of water weighs 1,000 g. How
many water molecules is that?
1,000 g H2O x 1 mole H2O x 6.02 x 1023 molecules H2O = 3.3 x 1023 molecules H2O
18.0148 g H2O
1 mole H2O
3.3 x 1023 molecules H2O  3 x 1023 molecules H2O
Correct Sig Fig
Practice Practice Practice!!!