The Mole, Avogadro’s Number & Molar Mass
Packet #20
COUNTING ATOMS BY WEIGHING
Counting Atoms by Weighing I
• C(s) + O2(g) CO2 (g)
• Suppose a scientist has a small pile of carbon and wants to know how many oxygen molecules are required to convert all of the carbon into carbon dioxide.
• What can the scientist learn from the chemical equation?
• The balanced equation illustrates that one diatomic oxygen molecule (O2) is required for each carbon atom.
Counting Atoms by Weighing II
• C(s) + O2(g) CO2 (g)
• To determine how many oxygen atoms are needed by the scientist, the scientist must know how many carbon atoms are present in his/her pile.
• To determine this, the scientist must count the number of atoms through the weight of the sample provided.
AMU to grams Conversion
• AMU
• Atomic mass unit
• 1 amu = 1.66 * 10-24 g
• The periodic table provides a list of the average amu for each atom
• Aka atomic mass • Why?
• The atomic mass, listed on the periodic table, is measured in amu’s.
• These amu’s can be converted into grams using the equation provided.
Problem Example #1A
• Calculate the mass, in amu, for 1000 neutral carbon atoms
• (1000 atoms) * (12.01
amu/atom)
• (1000 atoms) * (12.01
amu/atom)
• = 12,010 amu
Problem Example 1B
• Let’s assume that when the scientist weighed the pile of natural carbon, the amount weighed was 3.00 * 1020 amu. How many carbon atoms does the scientist have?
• Remember: -1 carbon atom = 12 amu
• Conversion factor
• Calculation
• 3.00 * 1020 amu * (1 carbon atom/12 amu)
• 3.00 * 1020 amu * (1 carbon atom/12 amu)
• 2.50 * 1019 carbon atoms
Problem Examples 2 - 5
• Calculate the mass, in amu, of a sample of aluminum that contains 75 atoms.
• Calculate the mass, in amu, of a sample that contains 23 atoms.
• Calculate the number of sodium atoms present in a sample that has a mass of 1172.49 amu.
• Calculate the number of oxygen atoms in a sample that has a mass of 288 amu.
THE MOLE
Introduction
• A mole is a counting unit
• Similar to a dozen
• Dozen = 12
• A mole is equal to 6.02 * 1023 items
• This is known as Avogadro’s number
Mole
6.02 * 1023 atoms
Avogadro’s number
Counting Unit
Introduction II
• How many cars (individual cars) are found in 1 mole of cars (group)?
• 1 mole of cars = 6.02 * 1023 cars
• How many cookies (individual cookies) are found in 1 mole of cookies (group)?
• 1 mole of cookies = 6.02 * 1023 cookies.
Introduction III
• Example III
• One mole of carbon (element)
• 1 mole C = 6.02 * 1023 atoms
• Remember
• An element is a substance composed of atoms that are all the same.
MOLAR MASS
Molar Mass an of Atom
• The molar mass of an atom is equivalent to the atom’s atomic mass.
Molar Mass Example 1A
• What is the molar mass of 1 mole of oxygen (mono)?
• 16 grams
Molar Mass Example 1B
• What is the molar mass of 3 mole of oxygen?
• 3 * 16 grams
Molar Mass Example 1C
• How many moles are found within 64 grams of oxygen?
• 4 moles
Molar Mass of a Molecule/Compound
• The molar mass of a molecule/compound is equivalent of sum of the atomic masses of the all the atoms within the molecule/compound.
Molar Mass of a Molecule/Compound II
• Calculate the molar mass of one mole of sulfur dioxide (SO2).
• 1 mole of sulfur = 32.07 g
• 1 mole of diatomic oxygen (16.00 * 2 ) = 32 g
• 1 mole of SO2 = 64.07 g
Molar Mass of a Molecule/Compound II
• Calcium carbonate (calcite), CaCO3 is the principal mineral found in limestone, marble chalk, pearls and the shells of marine animals such as clams.
a. Calculate the molar mass of calcite.
• 100.09 g
b. A certain sample of calcite contains 4.86 mol. What is the mass in grams of this sample?
• 486 g of CaCO3
Molar Mass of a Molecule/Compound III
• Juglone, a dye known for centuries, is produced form the husks of black walnuts. It is also a natural herbicide that kills off competitive plants around the black walnut tree but does not affect grass and other non-competitive plants. The molecular formula is C10H6O3.
Molar Mass of a Molecule/Compound IV
• Calculate the molar mass of juglone
• 174.1 g = molar mass of juglone
• A sample of 1.56 g of juglone was extracted from black walnut husks. How many moles of juglone does this sample represent?
• 8.96 * 10-3 mol juglone
Molar Mass of a Molecule/Compound V
• Isopentyl acetate, C7H14O2, the compound responsible for the scent of bananas, can be produced commercially. Interestingly, bees release about 1 ug (1 * 10-6 g) of this compound when they sting. This attracts other bees, which join the attack. How many moles and how many molecules of isopentyl acetate are released in a typical bee sting?
• 5 * 1015 molecules
REVIEW
Example #1
• How many amu are in 1 mole of carbon?
• 12 amu
• How many amu’s are in 2 moles of carbon?
• (12 amu / 1 mole )* (2 moles)
• (12 amu / 1 mole )* (2 moles) = 24 amu
• How many grams are in 2 moles of carbon?
• Already calculated: -2 moles of carbon = 24 amu
• 1 amu = 1.66 * 1024
• (1.66 * 10^24/1 amu ) * (24 amu)
• (1.66 * 10^24/1 amu ) * (24 amu) =