| The
mole concept
Francine
Taylor-Campbell, Contributor
 |
| Members
of Central High School quiz team, at TVJ, on Tuesday, January 30. - Ricardo Makyn/Staff
Photographer | Main
points Relative
atomic mass, molecular mass and formula mass are measured in atomic mass units
(a.m.u.), where 1 a.m.u. 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 a.m.u. 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 a.m.u., 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 have something in common, in that they 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*1023.
Thus,
the mole is that amount of substance which 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 avogadro's 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 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 is an independent contributor. | 
