|
The
mole concept
Francine
Taylor-Campbell, Contributor
MAIN
POINTS
- Relative
atomic mass, molecular mass and
formula mass are measured in atomic
mass units (amu) where 1 amu is
one-twelfth (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
The
mole is the unit of measurement in
chemistry. In the same way we have
measurements such as one dozen, which
represents 12, one pair which is equal
to two and one score which represents
20, one mole suggests that 6.02 x
1023 particles are present.
One
mole of different substances may have
different masses, just as one dozen
apples may have a different mass compared
to one dozen mangoes.
Example:
One mole of carbon = 12g of carbon
One
mole of magnesium = 24g of magnesium
They
both represent one mole but they have
different masses.
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*1023.Thus,
the mole is that amount of substance
that contains 6.02*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) represents one mole
and contains 6*1023 (L)
atoms
1
mole of oxygen atoms (O) = 16g and
has L atoms
1
mole of oxygen molecules (O2)
= 32g and has L molecules
1
mole of carbon dioxide (CO2)
= 44g and has L molecules
1mole
of Na+ ions = 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 contains Avogadros
number of particles (6.0 * 1023).
For
example, 8g CuO 1 mol CuO = 64 + 16
= 80g
80g
contains 6.0*1023 formula
units
8g
contains (6.0 * 1023)/10
= 6.0 * 1022 formula units.
Another
way to look at this is to first of
all determine the number of moles
in 8g of CuO.
#
mol CuO == 8/80 = 0.1 mol
Now
1 mol CuO contains 6*1023 units
0.1
mol CuO contains (6*1023) x 0.1 =
6.0 * 1022 units.
Francine
Taylor-Campbell teaches at Jamaica
College. Send questions and comments
to kerry-ann.hepburn@gleanerjm.com
|
