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By the end of this lesson the student is expected ……..
Lecture, No-6
1. To prepare standard solutions
2. To practice acid-alkali titrations
3. To Calculate the concentration of
volumetric analysis
4. To Write a laboratory report on volumetric
analysis experiment
Lecture Objectives
Estimation of equivalence point by measuring the
change in pH of the mixture
P. 3 / 74
The change in pH of the mixture during
titration can be followed by using
a pH meter or
a data-logger connected with a pH
sensor
Steps for estimating equivalence point
by measuring the change in pH of the
mixture
1. Fill the 25.0 cm3 pipette with 0.1 M HCl(aq).
Transfer 25.0 cm3 of the acid to a beaker
placed on a magnetic stirrer.
2. Put the electrode of a pH meter or a pH sensor
connected with a data-logger into the beaker of
acid.
3. Fill the burette with 0.1 M NaOH(aq). Make sure
that the jet is filled completely (no air bubbles
inside the jet).
4. Slowly add the alkali to the acid. The pH of
the mixture is measured immediately after
each addition of the alkali. P. 4 / 74
magnetic
stirrer
magnetic stirring
bar
0.1 M
NaOH(aq)
burette
pH
meter
electrode of pH
meter 0.1 M
HCl(aq)
Figure 19.5 The experimental set-up for the estimation of
equivalence point by pH measurement.
If the pH reading is plotted against the volume of the sodium hydroxide solution added, a titration curve is obtained.
There is a marked change in the pH at
the equivalence point of titration.
Learning tip
The pH of the mixture at the equivalence point
may NOT be 7 for some acid-alkali titrations.
p
H
Volume of NaOH(aq) added
(cm3)
pH of the mixture when the equivalence
point is reached
equivalence
point
volume of NaOH(aq)
required to reach the
equivalence point
Titration curve for the titration of 0.1 M sodium hydroxide solution
with 25.0 cm3 of 0.1 M hydrochloric acid.
Think about
Estimation of equivalence point by measuring the
change in temperature of the mixture
Neutralization between an acid and an alkali
is an exothermic reaction.
The reaction gives out heat.
Steps for estimating equivalence point by
measuring the change in temperature of the
mixture
1. Place 25 cm3 of 0.1 M HCl(aq) and a
thermometer
in a polystyrene cup.
2. Record the initial temperature of the acid.
3. Add 0.1 M NaOH(aq) slowly to the acid.
4. Record the temperature of the mixture
immediately after each addition of NaOH(aq).
Learning tip
The set-up for the experiment here is very similar
to the one shown in Figure 19.5. Except that:
• a polystyrene cup is used instead of a beaker.
• a thermometer is used instead of a pH meter.
equivalence point
volume of NaOH(aq)
needed to reach the
equivalence point
Tem
pera
ture
(°
C)
E
A
B
Volume of NaOH(aq) added (cm3)
A plot of the temperature change of the mixture against the volume
of NaOH(aq) added.
Calculations on volumetric analysis
Standardization
To standardize (i.e. to find the molarity of) a
solution of unknown molarity by titrating it with
a standard solution.
To determine the basicity of an acid by finding
out the number of moles of sodium hydroxide
required to neutralize one mole of the acid.
Basicity of an acid
P. 12 / 74
Class practice 19.6
Molar mass of a substance
To determine the molar mass of an acid, a
metal or a base by volumetric analysis.
Example 19.8
Example 19.9
19.3 Calculations on volumetric analysis
To determine the relative atomic mass of
an element by volumetric analysis.
Relative atomic mass of an element
P. 13 / 74
Number of molecules of water of crystallization
To determine the number of molecules of water
of crystallization in a formula unit of a hydrated
compound by volumetric analysis.
19.3 Calculations on volumetric analysis
Percentage by mass of a substance in a sample
100%
Key point
Percentage by mass of a substance in a sample
(%) =
mass of the substance (g) mass of the sample(g)
The percentage by mass of
anhydrous sodium sulphate in this
bottle is 99%.
Back titration
1. Direct titration
Add just the right amount of titrant (the
solution in the burette) to the solution in the
conical flask.
Two ways to perform a
titration
direct
titration
back titration
2. Back titration
Direct titrations are impossible in some cases.
E.g. percentage by mass of CaCO3(s) in a
sample cannot be found by titrating standard
HCl(aq) with the sample directly.
∵ CaCO3(s) is insoluble in water and it is
unable to make an aqueous solution of it.
Learning tip
Back titration is also performed if the
reaction between the substance being
analysed and the reagent involved is too
slow.
Steps for back titration 1. Add a known excess amount of standard
HCl(aq) to a known mass of the CaCO3(s)
sample.
CaCO3(s) reacts with HCl(aq),
dissolving completely to
form a solution.
2. Back titrated the unreacted acid in the
solution against a standard NaOH(aq).
Writing a laboratory report on volumetric analysis
Format of laboratory reports
1
Use the correct
tenses
You should use past tense to describe
specific experimental methods and
observations.
2
Write in the
third person
and passive
voice
Because scientific experiments demonstrate
facts that do not depend on the observer,
reports should not be written in the first or
the second person. Besides, passive voice
is used.
3 Write with
proper grammar
Complete sentences with no grammatical
mistakes should be used.
1
2
3
Title
Write the title of the experiment as given in the
laboratory manual.
(e.g. Determining the concentration of ethanoic
acid in commercial vinegar)
Objective
Use a single sentence to state clearly the objective
of the experiment.
(e.g. To determine the concentration of ethanoic
acid in commercial vinegar)
Apparatus
and
materials
List the apparatus and materials used in the
experiment as specifically as possible. This allows
others to repeat your experiments.
(e.g. electronic balance
beaker (100 cm3)
volumetric flask (250.0 cm3)
anhydrous sodium carbonate powder (6.30 g))
4
Procedure
• Describe clearly the procedure of the experiment
so that someone else can repeat what you did.
For example, describe clearly how solutions are
obtained or prepared and used in the analysis;
how measurements are done; state the amounts
of substances used, the volumes and
concentrations of solutions used.
• In general, write the procedure in a step-by-step
format rather than in essay form.
• In most cases, draw diagrams of the experimental
set-ups for better presentation.
5
Results and
observations
• Summarize your numerical data using tables or
graphs.
• Express numerical results to the appropriate
number of significant figures and in the correct
units.
• Record all observations clearly and fully using
appropriate terms in chemistry.
6
Analysis/
Calculations
(if any)
• Give the equations used to work out the
answers.
• Show your calculation steps clearly.
• Provide definitions of every variable used.
7
Conclusions
and
evaluation
• Draw valid conclusions based on the
observations and data collected.
• Describe the problems encountered in the
experiment.
• Discuss the sources of errors and uncertainties,
especially those due to error in laboratory
techniques and those due to imprecision of the
apparatus.
• Give any suggestions for improvement to avoid
these errors.
• Answer the questions (if any) in the laboratory
manual at the end of the laboratory report.
Attitude of writing laboratory reports
We should not just report ‘good’ data.
Try to find out why irregularities arise, whether
we have made some mistakes while performing
experiments or we have to revise the
experimental procedures.
Summary
Standard solutions
1. A standard solution is a solution of
accurately known molar concentration or
molarity.
2. The essential apparatus used in the
preparation of a standard solution
include volumetric flask, pipette and
pipette filler.
3. A primary standard is a substance which can
be used to prepare a standard solution
directly.
4. A standard solution can be prepared by
dissolving a known mass of a pure solid or
by diluting a concentrated standard
solution to a specific volume.
19.2 Acid-alkali titrations
5. Volumetric analysis is a method to determine
the amount (or concentration) of a substance
in a sample. Titration is one of the techniques
of volumetric analysis.
6. The essential apparatus used in titration
include burette, pipette and conical flask.
7. The equivalence point of an acid-alkali
titration is the point at which an acid and an
alkali have completely reacted with each
other.
8. The equivalence point of an acid-alkali
titration can be estimated by measuring the
change in pH and the change in
temperature of the mixture.
9. End point is the point at which the
indicator changes colour sharply.
Calculations on volumetric analysis
11. We can apply the concepts of concentration
of solution and acid-alkali titration results to
solve stoichiometric problems such as
(a) Standardization
(b) Molar mass of a substance
(c) Basicity of an acid
(d) Relative atomic mass of an element
(e) Number of molecules of water of
crystallization of a hydrated
compound
(f) Percentage by mass of a substance in
a sample
Writing a laboratory report on volumetric analysis
12. A laboratory report can help to communicate
the procedure and results of an experiment
on volumetric analysis.
13. A formal laboratory report should consist of
the following sections:
(a) Title
(b) Objective
(c) Apparatus and materials
(d) Procedure
(e) Results and observations
(f) Analysis and calculations (if any)
(g) Conclusions and evaluation
Concept map
apparatu
s determine
s
• c_o_nc_e_n_tr_a_t_io_n o_f_s_o_lu__ti_o_n
• basicity of an acid
• molar mass of a substance
• relative atomic mass of an
element
• number of molecules of
water of crystallization
• percentage by mass of a
substance in a sample
VOLUMETRIC
ANALYSIS
methos
Titration
VOLUMETRIC ANALYSIS
Preparation of a
_
Dissolve a known mass of a
Dilute a solution of
known molarity
example
Acid-alkali titration
detection
of end
point
detection of
equivalence
point
Titration
Measure the
change in _p_H
of the mixture
Measure the
change in
of the mixture