Chemistry Year 1
SECTION
2
THE CONCEPT OF THE
MOLES
,SECTION 2 THE CONCEPT OF THE MOLES
PHYSICAL CHEMISTRY
Matter and Its Properties
INTRODUCTION
In this section you will be introduced to fundamental concepts such as relative
atomic mass, relative molecular mass, and the mole as a unit of amount of
substance. You will also learn how to perform calculations based on the amount
of substance and understand the importance of the mole concept in preparing
standard solutions.
At the end of this section, you should be able to:
• Explain relative atomic mass and relative molecular mass.
• Describe the atomic mass unit as an average mass.
• Describe the mole as a unit of the amount of substance.
• Calculate different physical quantities (number of entities, mass and
volume) based on the amount of substance.
• Explain the mole concept and its relevance in preparation of standard
solutions.
Key Ideas
• Atomic mass unit (amu) is equal to 1/12th the mass of a carbon-12
atom.
• Relative atomic mass is the average mass of an element’s atoms,
considering isotopes.
• Relative molecular mass: the sum of the relative atomic masses of all
atoms in a molecule.
• Mole is a way of measurement in chemistry.
• Avogadro’s constant is number of units in one mole of any substance.
• Entities refers to any distinct atom, molecule, ion.
• Avogadro’s number is the number of particles in one mole.
2
,SECTION 2 THE CONCEPT OF THE MOLES
• Avogadro’s number is 6.022 × 10 23particles (atoms, molecules, and
ions)
• Mole (mol) is a unit of measurement.
• Molar mass is mass of mole a substance.
RELATIVE ATOMIC MASS (Ar)
Relative Atomic Mass (Ar) is defined as the average mass of one atom of the
element compared to 1/12th of the mass of one atom of carbon-12.
Mathematically Relative Atomic mass is represented as,
Average mass of one atom of the element
Ar = _____________________________
_
1 2 the mass of one atom of carbon − 12
1
th
This value is expressed in atomic mass units (amu). The relative atomic mass is
determined by the abundance of each isotope of an element.
Worked Example 2.1
One atomic mass unit of Carbon – 12 is 1.6603 × 10−24g. If the average mass of
an atom of oxygen is 2.65659 ×10 −23g. Determine its relative atomic mass.
Solution:
Ar(O)
Average mass of one atom of the element
= _____________________________
_
1
12 the mass of one atom of carbon − 12
th
2.65659 × 10−23 g
= ____________
1.6603 × 10 g
−24
= 16.0
Relative Molecular Mass (Mr)
Relative molecular mass is defined as the average mass of one molecule of a
substance compared with _
1 2 of the mass of one atom of carbon-12.
1
th
average mass of one molecule of the substance
_________________________________
Mr =
_
12 the mass of one atom of carbon − 12
1
th
It also has no unit. For ionic compounds, the relative molecular mass is called its
Relative formula mass (as ionic substances do not exist as molecules).
3
, SECTION 2 THE CONCEPT OF THE MOLES
Relative molecular mass is the sum of the masses of the elements that make up
the molecule.
Activity 2.1: Determining the mass of an element or compound using a
beam balance
Materials needed:
Beam balances, standard carbon-12 samples (represented by 12 beads or any
small, identical objects), samples of different elements or compounds (using
different numbers of beads/objects), worksheets for recording observations
and calculations, calculators
Steps:
Carry out this activity in small groups.
1. Set up the beam balance, get a standard carbon-12 sample (12 beads), and
samples of other elements or compounds (different numbers of beads).
2. Place the carbon-12 sample in one pan of the beam balance.
3. Place the unknown element or compound sample in the other pan of the
beam balance.
4. Adjust the number of beads/objects in the unknown sample until the
beam balance is level, indicating that the masses are equal.
5. Record the number of beads/objects used for the unknown sample to
balance the carbon-12 standard.
6. Calculate the relative mass of their unknown sample compared to
carbon-12.
Example Calculation:
If the unknown sample balanced with 22 beads, the relative mass
compared to carbon-12 (12 beads) is:
Average mass of one atom of the element
Ar = _____________________________
_
1 2 the mass of one atom of carbon − 12
1
th
So then as a balance is set up:
12 × (mass of 1 carbon atom) = 22 × (mass of 1 unknown atom)
So: as we know the mass of a carbon atom is 1 amu we can state that:
Number of beads in carbon − 12
Ar = __________________________
Number of beads in unknown sample 22
12 = 0.54
= _
4
SECTION
2
THE CONCEPT OF THE
MOLES
,SECTION 2 THE CONCEPT OF THE MOLES
PHYSICAL CHEMISTRY
Matter and Its Properties
INTRODUCTION
In this section you will be introduced to fundamental concepts such as relative
atomic mass, relative molecular mass, and the mole as a unit of amount of
substance. You will also learn how to perform calculations based on the amount
of substance and understand the importance of the mole concept in preparing
standard solutions.
At the end of this section, you should be able to:
• Explain relative atomic mass and relative molecular mass.
• Describe the atomic mass unit as an average mass.
• Describe the mole as a unit of the amount of substance.
• Calculate different physical quantities (number of entities, mass and
volume) based on the amount of substance.
• Explain the mole concept and its relevance in preparation of standard
solutions.
Key Ideas
• Atomic mass unit (amu) is equal to 1/12th the mass of a carbon-12
atom.
• Relative atomic mass is the average mass of an element’s atoms,
considering isotopes.
• Relative molecular mass: the sum of the relative atomic masses of all
atoms in a molecule.
• Mole is a way of measurement in chemistry.
• Avogadro’s constant is number of units in one mole of any substance.
• Entities refers to any distinct atom, molecule, ion.
• Avogadro’s number is the number of particles in one mole.
2
,SECTION 2 THE CONCEPT OF THE MOLES
• Avogadro’s number is 6.022 × 10 23particles (atoms, molecules, and
ions)
• Mole (mol) is a unit of measurement.
• Molar mass is mass of mole a substance.
RELATIVE ATOMIC MASS (Ar)
Relative Atomic Mass (Ar) is defined as the average mass of one atom of the
element compared to 1/12th of the mass of one atom of carbon-12.
Mathematically Relative Atomic mass is represented as,
Average mass of one atom of the element
Ar = _____________________________
_
1 2 the mass of one atom of carbon − 12
1
th
This value is expressed in atomic mass units (amu). The relative atomic mass is
determined by the abundance of each isotope of an element.
Worked Example 2.1
One atomic mass unit of Carbon – 12 is 1.6603 × 10−24g. If the average mass of
an atom of oxygen is 2.65659 ×10 −23g. Determine its relative atomic mass.
Solution:
Ar(O)
Average mass of one atom of the element
= _____________________________
_
1
12 the mass of one atom of carbon − 12
th
2.65659 × 10−23 g
= ____________
1.6603 × 10 g
−24
= 16.0
Relative Molecular Mass (Mr)
Relative molecular mass is defined as the average mass of one molecule of a
substance compared with _
1 2 of the mass of one atom of carbon-12.
1
th
average mass of one molecule of the substance
_________________________________
Mr =
_
12 the mass of one atom of carbon − 12
1
th
It also has no unit. For ionic compounds, the relative molecular mass is called its
Relative formula mass (as ionic substances do not exist as molecules).
3
, SECTION 2 THE CONCEPT OF THE MOLES
Relative molecular mass is the sum of the masses of the elements that make up
the molecule.
Activity 2.1: Determining the mass of an element or compound using a
beam balance
Materials needed:
Beam balances, standard carbon-12 samples (represented by 12 beads or any
small, identical objects), samples of different elements or compounds (using
different numbers of beads/objects), worksheets for recording observations
and calculations, calculators
Steps:
Carry out this activity in small groups.
1. Set up the beam balance, get a standard carbon-12 sample (12 beads), and
samples of other elements or compounds (different numbers of beads).
2. Place the carbon-12 sample in one pan of the beam balance.
3. Place the unknown element or compound sample in the other pan of the
beam balance.
4. Adjust the number of beads/objects in the unknown sample until the
beam balance is level, indicating that the masses are equal.
5. Record the number of beads/objects used for the unknown sample to
balance the carbon-12 standard.
6. Calculate the relative mass of their unknown sample compared to
carbon-12.
Example Calculation:
If the unknown sample balanced with 22 beads, the relative mass
compared to carbon-12 (12 beads) is:
Average mass of one atom of the element
Ar = _____________________________
_
1 2 the mass of one atom of carbon − 12
1
th
So then as a balance is set up:
12 × (mass of 1 carbon atom) = 22 × (mass of 1 unknown atom)
So: as we know the mass of a carbon atom is 1 amu we can state that:
Number of beads in carbon − 12
Ar = __________________________
Number of beads in unknown sample 22
12 = 0.54
= _
4
