MOLE
&
Avogadro
Number
No way to count atoms one by one.
However, we need to know what amount
of each reagent to take in order the rxn to
proceed completely.
Example:
Mg + S  MgS
How much of Mg & S to take in other
they react completely, with no unreacted
material?
From the equation:
1 atom Mg (i.e. 24 a.m.u.)
combines with 1 atom of S (i.e. 32 a.m.u.).
Mass ratio, 24:32, does not depend on
units: it will be the same in a.m.u. in any
other units, say, in gram.
Take 12 g Mg & 32 g S: they will react
completely.
12 g is the molar mass of Mg
32 g the molar mass of S.
i.e. 12 g Mg react w/ 32 g S.
Two units of mass:
atomic mass unit & gram:
how are they related to each other?
1 g = 6.02.1023 a.m.u.
in
H2 + F2  2HF
a.m.u.: 2 38
40
g:
2 38
40
2 g of H2 gas react with 38 g F2 gas
producing 40 g HF gas
Since they react 1:1 on atomic basis, they
must contain the same number of atoms,
exactly the same, that relates gram to
a.m.u.
This number is Avogadro number, NAv =
6.02.1023
This number of particles is called a
MOLE.
Mole is a counting unit (like dozen, gross,
etc.)
The equation:
Mg + S  MgS
tells us not only that Mg & S combine 1
atom:1 atom,
but also that they combine
1 mole Mg to 1 mole S.
Molar mass is
the mass of 1 mole.
Molar mass of an element in grams is numerically
equal to the atomic mass of the element (in a.m.u.)
Molar mass of an ionic compound is the sum of molar
masses of constituent elements in its FORMULA
UNIT.
Molar mass of a molecular compound is the sum of
molar masses of constituent elements in its molecule.