Transcript Calculating Formula Mass

Chemical
Measurements
Unit 4: Stoichiometry
Chapter 10 – The Mole
Overview
• Measurements are essential.
• Measuring the number of atoms involved in a
chemical reaction is impractical.
• The mole establishes a relationship between the
number of particles involved in a reaction and the
mass of that reactant or product.
• Mole relationships are used to determine %
composition, empirical and molecular formulae.
Measurements
• We use measurements all of the time!
• Atomic Mass is a measurement that we have
already used in class.
o
o
o
units for atomic mass = amu
atomic mass unit
Used to express masses of atoms on a relative
scale. We compare everything to carbon-12.
Particle location
Mass (g)
Mass
(amu)
Inside
nucleus
1.673X10-24 1.0073=1
Inside
Neutron
nucleus
1.675X10-24 1.0087=1
Outside
Electron
nucleus
7.109X10-28 0.0006=0
Proton
Atomic Mass & Formula Mass
• Atomic Mass
The weighted average of the masses of the
existing isotopes of an element
o Ex. Carbon C 12 amu
o
• Formula Mass
The sum of the atomic masses of all atoms in a
compound
o Ex. Carbon Dioxide CO2 44 amu
o
Calculating Formula Mass
• Methylene chloride (CH2Cl2) is used as a solvent in
paint strippers. What is the formula mass of
methylene chloride?
o
o
o
C: 1 atom x 12.01 amu
H: 2 atoms x 1.01 amu
Cl: 2 atoms x 35.45 amu
• Formula Mass = 12.01(1) + 1.01(2) + 35.45(2) =
83.92 amu
Calculating Formula Mass
• What is the formula mass for SO2?
o
o
S: 1 atom x 32.07 amu
O: 2 atoms x 16.00 amu
Formula Mass = 32.07 amu + 16.00 amu (2)=
64.07 amu
Calculating Formula Mass
• What is the formula mass for H2O2?
o
o
H: 2 atoms x 1.01 amu
O: 2 atoms x 16.00 amu
Formula Mass = 2.02 amu + 32.00 amu =
34.02 amu
Moving from AMU to grams
• The use of atomic mass units (amu) are impractical
in the chemistry lab where the preferred unit of
measurement is grams.
• Scientists needed to establish a relationship between
# of atoms and masses of atoms.
Moving from AMU to grams
• We are missing something!
• We don’t know how many atoms are necessary to
make up a mass in grams that is equal to an
element’s atomic mass
• We need some sort of conversion factor
The Mole
• Definition
The number of atoms of that element equal to the
number of atoms in exactly 12.0 grams of carbon12.
o Abbreviated mol (without the ‘e’)
o
• The number of atoms in one mole of atoms is
always the same!
o
6.02 x 1023 atoms
Same Number, Different Mass
Element # atoms/mol mass of 1 mole
Copper 6.02x1023 atoms
Cu 63.55 g
Mercury 6.02x1023 atoms
Hg 200.59 g
Sulfur 6.02x1023 atoms
S 32..07 g
Iron 6.02x1023 atoms
Fe 55.85 g
Molecules and Moles
• The number of molecules in any molecular
compound is 6.02 x 1023
• 1 mole of water (H2O) contains 1 mole of
water molecules but 3 moles of atoms.
• How many moles of Ca2+ and F- are in 1
mole of calcium fluoride (CaF2)?
o
1 mole of Ca2+ ions and 2 moles of F- ions
Avogadro’s Number
• 6.02 x 1023 is the quantity of items in a mole
is known as Avogadro’s Number
(abbreviated N) in honor of Amadeo
Avogadro.
Molar Mass
• Definition
o
The mass in grams of 1 mole of a substance
• We calculate molar mass the same way as
formula mass.
o
Atomic/Formula Molar Mass
o
Ex. Calcium 40 amu = 40.08 g
Calculating Molar Mass
• Methylene chloride (CH2Cl2) is used as a solvent in
paint strippers. What is the molar mass of methylene
chloride?
o
o
o
C: 1 atom x 12.01 amu
H: 2 atoms x 1 amu
Cl: 2 atoms x 35 amu
• Formula Mass = 12.01(1) + 1.01(2) + 35.45(2) = 84.95
amu
• Molar Mass = 84.95 g/mol
Same Number, Different Mass
Practice
• What is the formula and molar mass for propane,
C3H8?
– C: 3 atoms x 12.01 amu = 36.03 amu
– H: 8 atoms x 1.01 amu = 8.08 amu
Formula Mass = 36.03 amu + 8.08 amu = 44.11 amu
Molar Mass = 44.11 g/mol
Same Number, Different Mass
Practice
• What is the formula and molar mass for glucose,
C6H12O6?
– C: 6 atoms x 12 amu = 72 amu
– H: 12 atoms x 1 amu = 12 amu
– O: 6 atoms x 16 amu = 96 amu
Formula Mass = 72 amu + 12 amu + 96 amu = 180 amu
Molar Mass = 180 g/mol