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THE MOLE: FORMULAE AND CHEMICAL EQUATIONS - Chemistry
Notes Form 3
Mole Concept
What is a Mole?
Relative Atomic Mass (R.A.M) and Moles
Relative Formula Mass (R.F.M) of a Compound
Stoichiometric and Ionic Equations
Chemical Word Equations
Stoichiometric Equations
Ionic Equations
Molarity
Titration: Acid and Alkali
How to Carry Out a Titration
Volumetric Analysis Titrations
Titration Technique
Mole Concept
What is a Mole?
A mole is a word which represents a number. Like the word "dozen" represents the number 12, so
"mole" represents the number 6 x 1023.
This number is also called Avogadro's number.
It is a very very big number (6 followed by 23 zeros).
In the same way that you can have a dozen atoms, or cars, or apples, so you can also have a mole
of atoms, or cars, or apples.
Needless to say, chemists are concerned with atoms, ions, molecules and compounds. A mole is
defined as the number of atoms in exactly 12 grams of 12C (carbon twelve).
The number is called Avogadro's number. In the above definition, 12 is the mass number of
carbon. So, one mole of carbon atoms has a mass of 12 grams. The relative atomic mass, which
can be written as Ar or RAM, is the number just above the element in the periodic table
Relative Atomic Mass(R.A.M) and Moles
Just as one mole of carbon atoms has a mass of 12 g, so the mass of one mole of the atoms of any
element is its "relative atomic mass" in grams.
For example, look up the relative atomic mass of sodium (Na), (the larger number above it in the
periodic table).
One mole of sodium has a mass of 23 g.
One mole of helium has a mass of 4 g,
One mole of neon has a mass of 20 g,
One mole of magnesium has a mass of 24 g,
One mole of calcium has a mass of 40 g.
This is easy for elements which exist as atoms.
As discussed ealier, the number of protons added to the number of neutrons is known as the
relative atomic mass.
This is the mass of 1 mol of an atom relative to the mass of 1 mol of C atoms that have 6 protons
and 6 neutrons, which is taken to be 12.00 g.
However there are complications due to isotopes and so very accurate atomic masses are not
whole numbers.
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Isotopes are atoms of the same element with different masses due to different numbers of
neutrons.
12
The very accurate atomic mass scale is based on a specific isotope of carbon, carbon - 12, C=
12.0000 units exactly, for most purposes C = 12 is used for simplicity.
The strict definition of relative atomic mass (Ar) is that it is equals average mass of all the isotopic
atoms present in the element compared to 1/12th the mass of a carbon-12 atom.
Examples of relative atomic mass calculations
Example 1: Chlorine consists of 75% chlorine-35 and 25% chlorine-37. Calculate its relative atomic
mass
Solution: Think of the data based on 100 atoms, so 75 have a mass of 35 and 25 atoms have a
mass of 37.
The average mass = [ (75 x 35) + (25 x 37) ] / 100 = 35.5
So the relative atomic mass of chlorine is 35.5 or Ar(Cl) = 35.5
What about elements which exist as molecules or compounds?
Relative Formula Mass of a Compound (R.F.M)
To calculate the mass of one mole of a compound, the number of each type of atom in the
compound is multiplied by that atoms relative atomic mass and all those numbers added together.
This value is called the relative formula mass (or relative molecular mass or molar mass) of
a compound
NB: Notice that r.f.m, r.m.m, or Mr have no units because they are ratios. The molar mass is
obtained from r.f.m, r.m.m, or Mr by simply adding g (grams)
If all the individual atomic masses of all the atoms in a formula are added together you have
calculated the relative formula mass (for ionic compounds) or molecular mass (for covalent
elements or compounds).
Mr. can be used for any element or compound. Whereas relative atomic mass (above) only applies
to a single atom.
Anything with at least two atoms requires the term relative formula/molecular mass.
The most common error is to use atomic/proton numbers instead of atomic masses, unfortunately,
except for hydrogen, they are different!
Examples of formula/molecular mass calculations:
Example 1: Calculate the relative formula mass of the the diatomic molecules of the elements
hydrogen H2 and chlorine Cl2. Relative atomic masses, Ar: H = 1, Cl = 35.5
Solution: Formula masses, Mr, are H2 = 2 x 1 = 2, Cl2 = 2 x 35.5 = 71 respectively.
Example 2: Calculate the relative formula mass of the compound calcium hydroxide Ca (OH)2
(ionic). Relative atomic masses are Ca=40, H=1 and O=16
Solution: Mr = 40 + 2 x (16+1) = 74
Mass from amount:
The key mathematical equation needed here is:
Mass (g) = relative formula mass (g mol-1) x amount (mol)
Using the triangular relationship from above if the mass section is covered over then the amount
multiplied by the relative formula mass gives the mass.
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Examples
i. What is the mass of 0.25 mol of NaCl?
Solution: 0.25 mol of NaCl = 58.5 g mol-1 × 0.25 mol
= 14.63 g
ii. What is the mass of 3 mol of Al2(SO4)3?
Solution: 3 mol of Al2(SO4)3 = 342 g mol-1 × 3 mol
= 1026 g or 1.026 kg
Amount from Mass
The key mathematical equation needed here is
Amount (mol) = mass (g)/relative formula mass (g mol-1)
Using the triangular relationship from above if the amount section is covered over then the mass
divided by the relative formula mass gives the amount -
Example calculations
i. What amount is 117 g of NaCl?
Solution: 117 g of NaCl
= 117 g/58.5 g mol-1
= 2 mol
ii. What amount is 68.4 g of Al2(SO4)3?
Solution: 68.4 g of Al2(SO4)3
= 68.4 g/342 g mol-1
= 0.2 mol
Molar Mass from Mass and Amount
The key mathematical equation needed here is
Relative formula mass (g mol-1) = mass (g)/amount (mol)
Using the triangular relationship from above if the molar mass section is covered over then the
mass divided by the amount gives the relative formula mass.
Example calculations
What is the molar mass of a compound for which 0.2 mol of it has a mass of 42 g?
Solution: molar mass of compound= 42 g/0.2 mol
= 210 g mol-1
Every mole of any substance contains the same number of the defined species.
The actual particle number is known and is called the Avogadro Constant and is equal to 6.023 x
1023 'defined species' per mole.
This means there are that many atoms in 12g of carbon (C = 12) or that many molecules of water
THE MOLE: FORMULAE AND CHEMICAL EQUATIONS - Chemistry
Notes Form 3
Mole Concept
What is a Mole?
Relative Atomic Mass (R.A.M) and Moles
Relative Formula Mass (R.F.M) of a Compound
Stoichiometric and Ionic Equations
Chemical Word Equations
Stoichiometric Equations
Ionic Equations
Molarity
Titration: Acid and Alkali
How to Carry Out a Titration
Volumetric Analysis Titrations
Titration Technique
Mole Concept
What is a Mole?
A mole is a word which represents a number. Like the word "dozen" represents the number 12, so
"mole" represents the number 6 x 1023.
This number is also called Avogadro's number.
It is a very very big number (6 followed by 23 zeros).
In the same way that you can have a dozen atoms, or cars, or apples, so you can also have a mole
of atoms, or cars, or apples.
Needless to say, chemists are concerned with atoms, ions, molecules and compounds. A mole is
defined as the number of atoms in exactly 12 grams of 12C (carbon twelve).
The number is called Avogadro's number. In the above definition, 12 is the mass number of
carbon. So, one mole of carbon atoms has a mass of 12 grams. The relative atomic mass, which
can be written as Ar or RAM, is the number just above the element in the periodic table
Relative Atomic Mass(R.A.M) and Moles
Just as one mole of carbon atoms has a mass of 12 g, so the mass of one mole of the atoms of any
element is its "relative atomic mass" in grams.
For example, look up the relative atomic mass of sodium (Na), (the larger number above it in the
periodic table).
One mole of sodium has a mass of 23 g.
One mole of helium has a mass of 4 g,
One mole of neon has a mass of 20 g,
One mole of magnesium has a mass of 24 g,
One mole of calcium has a mass of 40 g.
This is easy for elements which exist as atoms.
As discussed ealier, the number of protons added to the number of neutrons is known as the
relative atomic mass.
This is the mass of 1 mol of an atom relative to the mass of 1 mol of C atoms that have 6 protons
and 6 neutrons, which is taken to be 12.00 g.
However there are complications due to isotopes and so very accurate atomic masses are not
whole numbers.
, Get more notes and past papers at www.easyelimu.com. (WhatsApp only +254703165909 for more)
Isotopes are atoms of the same element with different masses due to different numbers of
neutrons.
12
The very accurate atomic mass scale is based on a specific isotope of carbon, carbon - 12, C=
12.0000 units exactly, for most purposes C = 12 is used for simplicity.
The strict definition of relative atomic mass (Ar) is that it is equals average mass of all the isotopic
atoms present in the element compared to 1/12th the mass of a carbon-12 atom.
Examples of relative atomic mass calculations
Example 1: Chlorine consists of 75% chlorine-35 and 25% chlorine-37. Calculate its relative atomic
mass
Solution: Think of the data based on 100 atoms, so 75 have a mass of 35 and 25 atoms have a
mass of 37.
The average mass = [ (75 x 35) + (25 x 37) ] / 100 = 35.5
So the relative atomic mass of chlorine is 35.5 or Ar(Cl) = 35.5
What about elements which exist as molecules or compounds?
Relative Formula Mass of a Compound (R.F.M)
To calculate the mass of one mole of a compound, the number of each type of atom in the
compound is multiplied by that atoms relative atomic mass and all those numbers added together.
This value is called the relative formula mass (or relative molecular mass or molar mass) of
a compound
NB: Notice that r.f.m, r.m.m, or Mr have no units because they are ratios. The molar mass is
obtained from r.f.m, r.m.m, or Mr by simply adding g (grams)
If all the individual atomic masses of all the atoms in a formula are added together you have
calculated the relative formula mass (for ionic compounds) or molecular mass (for covalent
elements or compounds).
Mr. can be used for any element or compound. Whereas relative atomic mass (above) only applies
to a single atom.
Anything with at least two atoms requires the term relative formula/molecular mass.
The most common error is to use atomic/proton numbers instead of atomic masses, unfortunately,
except for hydrogen, they are different!
Examples of formula/molecular mass calculations:
Example 1: Calculate the relative formula mass of the the diatomic molecules of the elements
hydrogen H2 and chlorine Cl2. Relative atomic masses, Ar: H = 1, Cl = 35.5
Solution: Formula masses, Mr, are H2 = 2 x 1 = 2, Cl2 = 2 x 35.5 = 71 respectively.
Example 2: Calculate the relative formula mass of the compound calcium hydroxide Ca (OH)2
(ionic). Relative atomic masses are Ca=40, H=1 and O=16
Solution: Mr = 40 + 2 x (16+1) = 74
Mass from amount:
The key mathematical equation needed here is:
Mass (g) = relative formula mass (g mol-1) x amount (mol)
Using the triangular relationship from above if the mass section is covered over then the amount
multiplied by the relative formula mass gives the mass.
, Get more notes and past papers at www.easyelimu.com. (WhatsApp only +254703165909 for more)
Examples
i. What is the mass of 0.25 mol of NaCl?
Solution: 0.25 mol of NaCl = 58.5 g mol-1 × 0.25 mol
= 14.63 g
ii. What is the mass of 3 mol of Al2(SO4)3?
Solution: 3 mol of Al2(SO4)3 = 342 g mol-1 × 3 mol
= 1026 g or 1.026 kg
Amount from Mass
The key mathematical equation needed here is
Amount (mol) = mass (g)/relative formula mass (g mol-1)
Using the triangular relationship from above if the amount section is covered over then the mass
divided by the relative formula mass gives the amount -
Example calculations
i. What amount is 117 g of NaCl?
Solution: 117 g of NaCl
= 117 g/58.5 g mol-1
= 2 mol
ii. What amount is 68.4 g of Al2(SO4)3?
Solution: 68.4 g of Al2(SO4)3
= 68.4 g/342 g mol-1
= 0.2 mol
Molar Mass from Mass and Amount
The key mathematical equation needed here is
Relative formula mass (g mol-1) = mass (g)/amount (mol)
Using the triangular relationship from above if the molar mass section is covered over then the
mass divided by the amount gives the relative formula mass.
Example calculations
What is the molar mass of a compound for which 0.2 mol of it has a mass of 42 g?
Solution: molar mass of compound= 42 g/0.2 mol
= 210 g mol-1
Every mole of any substance contains the same number of the defined species.
The actual particle number is known and is called the Avogadro Constant and is equal to 6.023 x
1023 'defined species' per mole.
This means there are that many atoms in 12g of carbon (C = 12) or that many molecules of water
