Chem 400 Chapter 3 Lecture Notes Part 2
Molecular or Formula Masses, Avogadro&s Number, Moles, and Molar Masses:
¥ You learned that the number underneath an Elemental Symbol on the Periodic Table is its atomic
mass (the mass of 1 &average± atom in amu).
¥ What about a formula like (NH
4
)
3
PO
4
or C
6
H
12
O
6
?
How do we calculate the formula mass?
¥ It²s just the sum of all the atomic masses in the formula.
¥ OK that²s easy, but is a formula mass very useful in the lab?
¥ Here²s the balanced equation for the rxn of hydrogen gas with oxygen gas:
2H
2
(g) + O
2
(g)
&
2H
2
O(g)
¥ Reading this, we say: 2 molecules of hydrogen gas react with 1 molecule of oxygen gas to produce 2
molecules of water vapor.
¥ Can we weigh out individual molecules or ions on a lab balance?
¥ No, chemists weigh out compounds and elements in grams, which contain 10
23
molecules or atoms or
ions.
¥ So chemists needed a way to express large amounts of molecules easily without scientific notation.
¥ Well since chemists already had an amu system of atomic or formula masses, they thought that it
would be convenient to define a counting unit so that they could use the same mass for measuring
grams as well.
¥ This counting unit (just like a dozen is a counting unit equal to 12) is called the
mole
(abbreviated
mol) where:
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview.
Sign up
to
access the rest of the document.
 Fall '06
 Dr.Samples
 Atom, Atomic Mass, Mole, Periodic Table, Moles, Atomic mass unit, balancedÂ equation

Click to edit the document details