Analysis & Stoichiometry
Gordon Watson
Chemistry Department, Kelso High
School
Adv Higher Unit 2 Topic 1
Introduction
This topic explores various aspects of Chemical Analysis, leading to
an appreciation of the importance of Stoichiometry in chemical
reactions.
Stoichiometry
Stoichiometry involves the understanding of numerical relationships
between reacting substances.
One methane
molecule
CH
4
Two oxygen
molecules
2 O2
One carbon dioxide
molecule
CO
2
Two water
molecules
2 H 2O
The Mole
Molar relationships , in turn, allow us to establish measurable
relationships between reacting substances.
1 mole
moles
16g
32g
1 mole
moles
44g
25 l
50 l
25 l
2
2
36g
Volumetric Analysis
This method of chemical analysis involves
accurately measured volumes.
Instruments such as pipettes
and burettes are used to
measure volumes accurately
Solutions of unknown
concentration are titrated against
a solution of known
concentration - a primary
standard solution or standard
solution
Gravimetric Analysis
This method of analysis involves accurate weighing.
Access to an Analytical Balance,
capable of reading to 2 decimal
places at least, is essential.
The analysis will usually involve the
production of a suitable precipitate:very low solubility
high molecular mass
Stoichiometry
1 mole = gfm
Primary standard
A Primary standard is a
substance that has the following
characteristics:• a high purity (> 99.9%)
• is stable in air and in solution
• a reasonably high formula mass
• is reasonably soluble
Suitable substances include:
Potassium hydrogen pthalate (acid) and sodium carbonate
Standard Solution
Stoichiometry
Stoichiometry
n = mass / gfm
C = n / V
Standard Solution
A Standard Solution is one
whose concentration has been
established by titrating against a
Primary Standard …..
or against another Standard
Solution whose concentration had
been established by titrating
against a Primary Standard …..
Stoichiometry
C1V1 p1 = C2V2 p2
Dilutions
Once prepared,
standard solutions can
be used a stock
solutions and further
diluted solutions can be
made.
Stoichiometry
C1V1 = C2V2
Titrations
Acid-Base Titrations - neutralisation reactions requiring an
indicator to detect the end-point
NaOH(aq) + CH3COOH(aq)  NaCH3COO(aq) + H2O(l)
Redox Titrations - based on redox reactions, often self-indicating
due to strong colours, e.g. KMnO4
MnO4-(aq) + 8H+(aq)  Mn2+(aq) + 4H2O(l)
2I-(aq)
 I2(aq) + 2e-
Complexometric Titrations - based on ligand reactions, requiring
an indicator that can be replaced
[Ni(In)](aq) + EDTA4-(aq)  [Ni(EDTA)]2-(aq) + In
Acid-Base Titrations
Equivalence point
The equivalence point is when the reaction is just
completed
For a titration between a
strong acid (e.g HCl) and a
strong base (e.g NaOH) the
equivalence point will be
when pH = 7.
However, not all indicators
will complete their colour
change at this point so endpoint observed may be
different.
Indicators
Indicators change colour
over a pH range.
End point
In this case both indicators
would change just before or
just after the equivalence
point
In this case one indicator
would change just after the
equivalence point, but the
other would be no good.
Redox Titrations
An excess of MnO4- must be added to detect the end-point.
Fortunately MnO4- is so strongly coloured that end-point is very
close to equivalence point.
Complexometric Titration
Murexide indicator forms
a yellow2+
green complex with Ni ions.
EDTA is added and
starts to complex
2+
with any free Ni ions first.
Finally, EDTA will replace the murexide molecules and the
colour of free murexide - purple - will be produced.
Any decision about the end-point relies on there being enough
free murexide to produce a distinct colour change.
What about the equivalence point?
Difficult Titrations
The weaker the acid, the
smaller the region of rapid pH
change which includes the
equivalence point.
For very weak acids, it is
impossible to detect an endpoint close to the equivalence
point
Back Titration
The solution to this problem is a technique known as a back
titration.
A carefully measured volume of base would be added to
ensure complete reaction of the weak acid.
A strong acid would then be used to determine the excess
base left over.
The amount of base which reacted with the weak acid can now be
calculated and, hence, the amount of weak acid present originally.
Analysis &
Stoichiometry
End of Topic 1