The mole concept
Why did people start using
kilograms, meters, liters etc?
A mole is simply a unit of measurement.
Units are invented when existing units are
inadequate.
Chemical reactions often take place at levels
where using grams wouldn't make sense, yet
using absolute numbers of
atoms/molecules/ions would be confusing, too.
So how much is a mole?
A mole is the amount of a substance which contains
the same number of chemical species as there are
atoms in exactly 12 grams of the isotope carbon –
12.
In short – 1 mole = 6.02 x 1023 particles
This is called Avogadro’s constant
(more on him in a second)
The mole is a SI unit with the symbol mol.
(The word comes from the Latin for heap or pile)
Avogadro’s Constant (L)
Lorenzo Romano Amedeo Carlo
Avogadro di Quaregna e di Cerreto,
Count of Quaregna and Cerreto (9
August 1776, Turin, Piedmont – 9
July 1856) was an Italian scientist.
He is most noted for his contributions to molecular
theory, including what is known as Avogadro's law.
In tribute to him, the number of elementary entities
(atoms, molecules, ions or other particles) in 1 mole of
a substance, 6.02214179(30)×1023, is known as the
Avogadro constant.
Avogadro’s Constant (L)
Avogadro's number is the number of
particles found in one mole of a substance.
– Number of atoms in 1 mole of an element
– Number of molecules in 1 mole of a
compound
In the periodic table
The relative atomic mass in the periodic table is the mass of 1
mole of that atom.
So, in the case of Hydrogen, 1 mole weighs 1.01 g.
When working with molecules, the composition is given by its
molecular formula.
- The relative molecular mass (Mr) is calculated by adding the
relative atomic masses of the atoms that make up the
molecule
(What would the Mr of water be??)
Molecular mass of Water
So how do you calculate the number
of moles?
Example
Problem
• Determine the number of moles of CO2 in 454
grams.
Solution
First, look up the atomic masses for carbon and
oxygen from the Periodic Table.
The atomic mass of C is 12.01 and the atomic
mass of O is 16.00.
Manipulating the equation
Here’s an easy way to figure out how to use the
equation if mass or molar mass is asked instead
of number of moles:
Using Avogadro’s constant
Example
Calculate how many molecules are present in
3.00 moles of H2O.
What is given:
•n= 3.00 moles
•L= 6.02 x 1023
What is asked:
•N – number of particles.
References
• Brown, C & Ford , M (2009) Chemistry,
Pearson, Malaysia.
• http://chemistry.about.com