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