Chemistry

1. Relative Atomic Mass,
Average Atomic Mass,
Mole and Avogadro’s
Number
STOICHIOMETRY

2.  In stoichiometry, you will be looking at the amounts of substances
before the reaction and the amount of material that is produced by the
reaction. Stoichiometry is all about number. In solving problems
involving stoichiometry, you might be looking at the mass of the
reactants, mass of the products, chemical equations, molecular weights
and formulas.
 Atomic mass is the mass of an atom of a chemical element expressed in
atomic mass units. It is approximately equivalent to the number of
protons and neutrons in the atom (the mass number) or to the average
number allowing for the relative abundances of different isotope. Atomic
mass is expressed in amu or atomic mass units.
 Relative atomic mass is the mass of any atom obtained by comparing it
with the atomic mass of carbon-12.

3.  Average atomic mass is equal to the weighted average of all isotopes of
an atom. Most elements occur in nature as a combination of different
isotopes. Average atomic mass is dependent on both the mass and the
relative abundance of each of the element’s isotope. It can be computed
using the formula:

4.  Mole, abbreviated as mol, is the SI unit of material quantity. One mole is the
number of atoms in precisely 0.012 kg of carbon – 12. This means that the
unit mole, basically tells you how many atoms a substance has. A
representative particle can be an atom, an ion, a molecule or a formula unit.
The representative particle “atom” is used for elements. The representative
particle “molecule” is used for covalent compounds and for diatomic
elements. Diatomic elements are elements that consist of two atoms.
Diatomic elements include hydrogen, nitrogen, oxygen, fluorine, chlorine,
bromine and iodine. The representative particle “formula unit” is used for
ionic compounds.
 One mole of a substance contains 6.02 x 1023 representative particles. The
number 6.02 x 1023 is known as Avogadro’s number. For example:
1 mole of iron contains 6.02 x 1023 atoms of iron
1 mole of H2O contains 6.02 x 1023 molecules of H2O
1 mole of NaCl contains 6.02 x 1023 formula units of NaCl