SCH 3U1
THE MOLE - CHAPTER 5
When a new compound is discovered, the formula of the compound is of great
interest to scientists. When a chemical reaction is studied, scientists ask
themselves how much of each substance is required for this reaction. The SI unit
for amount of substance is the mole.
Chemists do not count out atoms or molecules because they are too small.
Measuring the mass is done instead. Thus, it is important to obtain a relationship
between _______________ and ___________________________________.
One helium atom has a mass of 4u (2p+and 2n0) and one nitrogen atom has a
mass of 14u (7p+and 7n0). "(“u”" is the short form for "atomic mass units)" If you
compare the same number of particles of each element, their ratio will always remain
the same 4:14 or 2:7.
It takes 6.02 x 1023 atoms (that is a lot of atoms!!! ----- 602 000 000 000 000
000 000 000) in order to have enough mass to equal 12.0000 g of the carbon-12
isotope. All other masses are compared to the carbon-12 isotope and an equal
number of particles (6.02 x 1023) are recorded on the periodic table. All masses are
a ratio with carbon-12.
The number (N) 6.02 x 1023 is called Avogadro's number in honour of the
Italian physicist, Amadeo Avogadro. One atom of an element is measured in atomic
mass units (u) and is called atomic mass. One mole of a substance is equal to 6.02 x
1023 particles and is measured in grams. This mass is called the molar mass.
* Using the periodic table, complete the following chart.
ELEMENT
H
ATOMIC MASS
1.0 u
MOLAR MASS
1.0 g
# PARTICLES
6.02 x 1023
C
__________
__________
_____________
O
__________
__________
_____________
For a covalently bonded molecule such as CO2, one particle has a mass that can
be obtained by adding the mass of each atom in the molecule. If that molecular
mass is scaled up by Avogadro's number the molar mass is measured in grams.
MOLECULE MOLECULAR MASS
MOLAR MASS
# MOLECULES
CO2
_____________
_____________
_____________
CCl4
_____________
_____________
_____________
N2
_____________
_____________
_____________
An ionically-bonded material is made up of cations and anions. Why is sodium
chloride not a molecule?
______________________________________________________________
______________________________________________________________
In the case of ionically-bonded material, the term used is not molecular mass
but formula mass.
COMPOUND
FORMULA MASS
MOLAR MASS
# OF +VE/-VE IONS
NaCl
___________
____________
___________
CaO
___________
____________
___________
SnF4
___________
____________
___________