Download What is a mole? - Chemical Paradigms

Lesson 1
Introduction to the Mole Concept and Avogadro’s constant
What is a mole?
Equipment needed:
 3 cups each containing the same number of a different object
 An electronic balance
Procedure:
Make sure steps are shown in calculations.
(1)
You have in front of you 3 cups each containing _____ objects
In cup 1 you have one mole of _________________________________
In cup 2 you have one mole of _________________________________
In cup 3 you have one mole of _________________________________
What is one mole?
(2)
On the electronic balance weigh each ‘cup’ and determine the mass for
each mole.
For cup 1 one mole of _______________ has a mass of _________ ±________g
For cup 2 one mole of _______________ has a mass of __________±________g
For cup 3 one mole of _______________ has a mass of __________±________g
(3)
Determine the mass of 2 moles of cup 1?
(4)
Determine the mass of 4 moles of cup 2?
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Lesson 1
(5)
Determine the mass of 6 moles of cup 3.
(6)
Determine the number of moles in 6.0 grams of cup 1.
(7)
Determine the number of moles in 6 grams of cup 2.
(8)
If 3.0 moles of cup 3 had a mass of 99 g, determine the mass of one mole of cup 3?
(9)
Consider the following symbols: M = mass of one mole, m = mass of substance, n = number
of moles. Deduce a formula linking n, m and M. Make n the subject of the formula.
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Lesson 1
Avogadro’s constant
In Chemistry the word mole is used to describe the amount of a substance. A mole is like a packet
of particles. Each packet contains one mole or 6.02 x 1023 mol-1 of particles. These particles can
be atoms, ions, compounds or molecules. Chemists call this number Avogadro’s number (L).
1 mole
=
Amount of a
substance (L)
6.02 x 1023 particles
=
There can be amounts of elements, molecules, compounds, electrons and ions. An element is
defined as a pure substances made up of atoms of the same type. For example magnesium, (Mg) is
an element because it is only of magnesium atoms. A molecule is defined as two or more nonmetal atoms joined together with a covalent bond. The non-metal atoms can be the same or
different. Carbon dioxide (CO2), hydrogen (H2) and methane (CH4) are all molecules. There are two
types of compounds. Those that are made of two or more different metal and non-metal atoms
joined with ionic bonds and those with two or more different non-metal elements joined with
covalent bonds. Glucose (C6H12O6), sodium chloride (NaCl), carbon dioxide (CO2), calcium
carbonate (CaCO3) and methane (CH4) are all compounds.
Mole ratios in chemical equations
We have learned that chemical equations show how elements and compounds interact with each
other. They are essentially before and after sentences that tell a story. Like any story you need to
be able to read before you understand what the words mean.

Methane + oxygen
carbon dioxide
|--------------reactants-------------|
CH4 (l)
+
2O2 (g)
+
water
+
energy
|-------------------------products--------------------|

CO2 (g) +
2H2O (g) +
energy
The word equation and chemical equation using formulae shows how chemists represent a
chemical reaction using words. It reads:
‘One mole of methane reacts with two moles of oxygen gas to produce [or yield] one mole of
carbon dioxide gas and two moles of water. Heat energy is released.’
The arrow represents produces or yield. Scientists never put an equal (=) sign instead of the arrow.
The substances reacting are called reactants and the substances formed are called products. The
(l) and (g) represent the state of the substance. (l) is for a liquid, (g) a gas, (s) a solid and (aq) for an
aqueous solution. An aqueous solution is formed when a solid compound is dissolved in water. For
example sodium chloride solution is formed by taking a known amount of solid sodium chloride
and dissolving it in a known volume of water. More about this later.
In a chemical reaction substances need to combine in the correct mole ratio in order for the
reaction to occur. Even if the number of moles changes the ratio of moles of reactants and
products does not change. In the reaction below two moles of hydrochloric acid reacts with one
mole of zinc.
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Lesson 1
Hydrochloric acid
2HCl(aq)
+
zinc
+
Zn(s)

zinc chloride

ZnCl2(aq)
+
hydrogen
+
H2(g)
Applying the mole ratio:
 2 moles of hydrochloric acid react with _____ mole of zinc to produce 1 mole of zinc
chloride and _____ mole of hydrogen.
 1 mole of zinc chloride contains 1 mole of zinc ions and 2 moles of chloride ions
 2 moles of hydrochloric acid contains ___ mole of hydrogen atoms and ___ of chlorine
atoms.
All chemical reactions MUST be balanced. The numbers and types of atoms before (the reactants)
and after (the products) MUST be the same! When this happens the total mass of the atoms
before and after the reaction will be the same. This is called Law of conservation of mass.
Counting Moles
C6H12O6 contains:
 1 mole of C6H12O6 molecules

6 moles of C atoms

12 moles of H atoms

6 moles of O atoms

24 moles of atoms in total
2 C6H12O6 contains:
 2 mole of C6H12O6 molecules

2 x 6 = 12 moles of C atoms

2 x 12 = 24 moles of H atoms

2 x 6 = 12 moles of O atoms

48 moles of atoms in total
6 Ne atoms contain
 6 x 1 = 6 moles of the Neon atoms
3 BF3 contains:
 ________ mole of BF3 molecules

________ moles of B atoms

________ moles of F atoms

________ moles of atoms in total
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Lesson 1
4 Ca(NO3)2 contains:
 ________ mole of the compound Ca(NO3 )2

________ moles of Ca atoms

________ moles of N atoms

________ moles of O atoms

________ moles of atoms in total
To calculate how many or the number (symbol N ) of particles (compound, atom, ion, molecule,
electron) the following formula is used:
Number or How many particles, N = moles, n x 6.02 x 1023
Where n represents the number of moles (amount of the substance) measured in mol.
To calculate the number of moles the formula can be rearranged to give:
n = N
6.02
1023
Therefore 3 mol of BF3 contains:
 3 x 6.02 x 1023 molecules of BF3
 3 x (1 x 6.02 x 1023) moles of B atoms
 3 x (3 x 6.02 x 1023) moles of F atoms
 3 x (4 x 6.02 x 1023) moles of atoms in total
Example 1:
Find the number of sulphur atoms in 4.0 moles of sulphur, S
N (S) = (4.0 x 1) x 6.02 x 1023
= 2.4 x 1024 atoms (2SF)
Example 2:
Calculate the number of hydrogen atoms in 0.3 moles of water, H 2O
N (H) = (0.3 x 2 ) x 6.02 x 1023
= 3 x 1023 atoms (1SF)
Example 3:
Calculate the number of moles in 2.5 x 1023 molecules of sulfuric acid, H2SO4
n (H2SO4) = N
= 2.5 x 1023
6.02 x 1023 6.02 x 1023
= 0.42 mol of H2SO4 (2SF)
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Lesson 1
Questions
Points are awarded for showing your reasoning, so all problems need to show your thinking step by
step. Write answers involving Avogadro’s constant in scientific notation and remember no naked
numbers – every value should have a unit.
Steps:
1.
2.
3.
4.
5.
6.
Identify the variables provided in the problem and their units.
Identify the unknown – what you are solving for and its units.
Write the relevant formula with the subject. e.g n(NaCl).
Substitute the known variables into the formula, including units.
Calculate the answer.
Make sure the answer has the correct number of significant figures and units.
1.
Outline the conventions used to write chemical equations.
2.
Give an example of substance that can be called both a
a) Molecule and a compound
b) Molecule and an element
c) Compound only
3.
Determine the number of:
a)
Helium atoms in 1.5 mol of helium atoms, He
b)
Oxygen atoms in 0.0050 mol of oxygen molecules, O2
c)
Ammonia molecules in 2.5 mol of ammonia molecules
d)
Oxygen atoms in 4.0 moles of glucose molecules, C6H12O6
e)
Oxygen atoms in 0.50 moles of carbon dioxide molecules
f)
Hydrogen atoms in 0.25 moles of ammonia molecules
g)
Chloride ions are there in 4.0 moles of CaCl2
h)
Molecules in 6.0 moles of hydrogen sulfide, H2S
i)
Gold ions in 0.30 moles of gold (III) chloride
4.
Determine the number of moles in:
a)
30.0 x 1023 atoms of neon
b)
0.60 x 1023 molecules of carbon dioxide
c)
3.6 x 1023 molecules of hydrogen
d)
1.5 x 1023 sodium ions
5.
A sample of phosphoric acid, H3PO4 contains 1.2 x 1023 molecules. Calculate:
a)
How many moles of phosphoric acid is this?
b)
How many atoms of phosphorus, hydrogen and oxygen atoms will there be in this
number of moles of phosphoric acid?
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