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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.
Ex 1
Find the formula mass of O2
Ex 2
Find the formula mass of CO2
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
17.2 Molar Mass
• The mass (in grams) of one mole of a
compound is called its molar mass.
Solving Problems
What is the molar mass of one mole of
CaCO3?
1. Looking for:
–
… molar mass of CaCO3
2. Given
–
… chemical formula
3. Relationships:
–
no. amu in formula = molar mass in grams
Solving Problems
4. Solution
Formula mass CaC03 = 100.09 amu
1 mole CaC03 = 100.09 g CaCO3
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
Elements occur in Nature as isotopes.
 1 oxygen atom = 16.0grams
 1mole of oxygen atoms=6.02x1023atoms=16.0grams
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.