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The
mole concept
Francine
Taylor-Campbell, Contributor
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Calabar
versus St. Catherine High in
last year's Manning Cup football
action. Calabar won 2-1. - Ian
Allen Photo
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POINTS
TO NOTE
Relative atomic mass, molecular mass
and formula mass are measured in atomic
mass units (amu), where one amu is
1/12 of the mass of a carbon-12 atom,
12C.
The relative atomic mass of an element,
AR, is the average mass in amu of
all of its atoms. These values are
generally given in tables.
The relative molecular mass of a compound,
Mr, is the average mass of all of
its molecules in amu, while the relative
formula mass, Mr, is the average mass
of the formula units and applies specifically
to ionic compounds.
Molar mass is the mass of one mole
of the substance. The values of Ar
and Mr stated in grams are the molar
masses.
THE
MOLE
12g
of carbon and 24g of magnesium all
represent one mole of a substance.
The
mole is the amount of substance in
its molar mass. One mole of a substance
always contains the same number of
atoms, ions, molecules or formula
units as one mole of any other substance.
This is called the avogadro number
or avogadro constant (L). Its value
is 6.02 x 1023.
Thus the mole is that amount of substance
that contains 6.02 x 1023
particles of that substance. It is
usually given in grams but could be
given in kilograms or dm3
for gases.
12g
of carbon (C) represent one mole and
contain 6 x 1023
(L) atoms one mole of oxygen atoms
(0) = 16g and have L atoms
1
mole of carbon dioxide (CO2)
= 44g and has L molecules
1
mole of Na+-
ion = 23g and has L ions.
To
calculate the number of moles in a
substance the following formula is
used:
| #mol
== |
mass
of substance(g)
|
| |
mass
of one mole (g/mol) |
For
example to calculate the number of
moles in 60g of calcium we would divide
by the Mr of calcium. #mol of Ca ==60/40
= 1.5 mol
The
formula can be manipulated to calculate
an unknown mass given the number of
moles, in which case, mass of substance
== #mol * mass of one mole (Mr).
Therefore
the mass of 0.2 mol of NaOH is
Mass
of NaOH == 0.2 * (23 + 16 + 1) = 0.2*40
= 8g
To
calculate the number of atoms, molecules
or formula units present in a substance
we make use of the fact that one mole
of any substance contain avogadro,
s number of particles (6.0 x 1023)
For
example, 8g Cu0 1 mol Cu0 = 64 + 16
= 80g
80g
contains 6.0 x 1023
formula units
8g
contains (6.0 x 1023)/10
= 6.0 x 1022 formula
units.
Another
way to look at this is to first of
all determine the number of moles
in 8g of Cu0.
#mol
Cu0 ==8/80 = 0.1 mol
Now
1 mol Cu0 contains 6 x 1023
units
0.1
mol Cu0 contains (6 x 1023)
x 0.1 = 6.0 x 1022
units
*
Francine Taylor-Campbell is
an independent contributor.
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