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FORMULA MASS
17.2 Conservation of mass
 Lavoisier showed that
a closed system must
be used when studying
chemical reactions.
 When chemicals are
reacted in a closed
container, you can
show that the mass
before and after the
reaction is the same.
17.2 Formula mass
 The sum of the atomic mass values of the
atoms in a chemical formula is called the
formula mass.
Atomic & Molecular Weights
Chemical formulas and chemical equations have quantitative
significance.
H2O means 2 atoms of hydrogen and 1 atom of oxygen.
C3H8 + 5O2 → 3CO2 + 4 H2O
means 5 moles of oxygen gas
Definitions:
 Molecular Formula: the actual number of atoms
of each element in the compound.
 Molar mass: the mass of one mole of any
substance
 Atomic mass unit: amu
 The number of amu’s in formula mass =
molar mass in grams
•
17.2 Molar Mass
Example
 Formula mass CaC03 = 100.09 amu
 1 mole CaC03 = 100.09 g CaCO3
 Which is the Molar Mass
17.2 Avogadro’s Number
 The Avogadro number was named in honor of
Amedeo Avogadro who discovered that a mole
of any gas under the same conditions has the
same number of molecules.
 Johann Josef Loschmidt, a German physicist,
named and discovered the Avogadro number.
 Loschmidt realized that a mole of any
substance—be it a gas, liquid, or solid—
contains 6.02 x 1023 atoms or molecules.
Avogadro’s Number
 Avogadro’s number states that for one mole of any
substance, whether element or compound, there 6.02x1023
particles present in the sample.
 It is also the number of molecules in the formula mass of
a compound when these masses are expressed in grams.
 One set of 6.02x1023 atoms or molecules is also referred
to as a mole of that substance.
Remember
1 gram = 6.02214 x1023 amu
The atomic mass unit(amu) is assigned by
assigning exactly 12 amu to the 12C isotope
 1 oxygen atom = 16.0grams
 1mole of oxygen atoms=6.02x1023atoms=16.0grams
 What Is a Mole?
https://www.youtube.com/watch?v=TEl4jeETVmg
Formula Weight & Molecular Weight
Formula Weight – Ionic Compounds
Molecular Weight – Molecular/Covalent Compounds
****We tend to use interchangeably****
To find the Formula/Molecular Weight
Simply find the sum of the atomic weights of each
atom in the compound.