CCEA ADVANCED SUBSIDIARY

CHEMISTRY

MODULE 1
 
1.9   Titrations

 

Volumetric Analysis (Titrations)

 

A titration is a laboratory procedure where a measured volume of one solution is added to a known volume of another reagent until the reaction is complete.  The operation is an example of volumetric (titrimetric) analysis. The equivalence point is usually shown by the colour change of an indicator and is known as the end-point.

Volumetric analysis is a powerful technique, which is used in a variety of ways by chemists in many different fields.

 
Practical Aspects

 

The practical aspects of titrations are required in the assessment of practical skills.

A knowledge of the techniques of titrations is expected but it would be normal to assume that all apparatus would have been washed with distilled/deionised water. The description should include which reagent is placed in the burette, name of indicator (but no reason for choice of indicator), detection of endpoint and what should be observed, repetition for accuracy.

 

When you have finished this section you should be able to:

·         Perform titrations

·         Record titration results in the form of a table

 

 

Use of a VOLUMETRIC FLASK

To prepare a solution of precisely known concentration (a standard solution), a definite amount of solute must be dissolved in a solvent to give a definite volume of solution.  The definite amount of material is measured by weighing, and the definite volume of solution prepared in a volumetric flask. A volumetric flask contains a definite volume when correctly filled to the calibration mark at the temperature stated on the flask. Tip the solid from a weighing bottle into a large (250 cm3) beaker and add about 50 cm3 of distilled water from a wash bottle. Stir well with a glass rod to dissolve. Take great care not to lose any of the solution and remember to wash the solution off the stirring rod back into the beaker. Rinse out the volumetric flask with distilled water and pour the cold solution into the flask through a clean filter funnel. Wash out the beaker several times and add all the washings to the flask. Now fill the flask to within about 1 cm of the calibration mark on the neck. Finally add water dropwise until the meniscus just rests on the calibration mark. Stopper the flask and invert a number of times to thoroughly mix the contents.

Use of a PIPETTE

The pipette is designed to deliver a definite fixed volume of liquid when correctly filled to its calibration mark. Before use a pipette must be washed out with the solution it is to measure. To fill the pipette, use a safety filler to suck solution up a few centimetres above the calibration mark. Let the solution down until the bottom of the meniscus just touches the calibration mark. For a titration the contents of the pipette are run into a conical flask, which has been well washed with distilled water. Allow the pipette to drain for about 20 seconds, then touch the tip to the surface of the liquid in the conical flask. The volume of solution delivered by the pipette is known as the aliquot.

Use of a BURETTE

The burette is designed to deliver definite but variable volumes of liquid. First rinse out the burette with the solution it is to contain. Clamp the burette vertically in a stand. Fill the burette carefully using a beaker and a filter funnel. Open the tap briefly to fill the burette below the tap making sure there are no trapped air bubbles. Read the burette scale by observing the position of the bottom of the liquid meniscus, making sure your eyes are level with the graduation mark. To observe the meniscus more clearly, hold a white card behind the burette. Record the volume reading to the nearest 0.05 cm3.

 

Titration TECHNIQUE

When performing a titration, place the conical flask containing the aliquot on a white tile under the burette so that the tip of the burette is inside the mouth of the conical flask. Add a few drops of a suitable indicator to the solution in the conical flask. First perform a ‘rough’ titration by taking the burette reading and running in the solution in approximately 1 cm3 portions, while swirling the flask vigorously. When the end-point is reached, as shown by the indicator changing colour, quickly close the tap. The new burette reading will give you a rough idea (to within about 1 cm3) of the volume to be added. Now repeat the titration with a fresh aliquot. As the rough end-point volume is approached, add solution from the burette one drop at a time until the indicator changes colour. Record the volume. The volume run out from the burette to reach the end point is known as the titre.

 

Recording Titration Results

The results of a titration should be recorded;

·         Immediately

·         In ink

·         In a table

·         To the correct number of decimal places

 

Record the titration results in the form of a table.

 

Pipette solution

 

mol dm-3

cm3

Burette solution

 

mol dm-3

 

Indicator

 

 

 

 

Trial

1

2

3

(4)

Burette readings

Final

 

 

 

 

 

 

Initial

 

 

 

 

 

Volume used (titre) /cm3

 

 

 

 

 

Mean titre  /cm3

 

 
Accuracy

Record burette readings to the nearest 0.05 cm3 (approximately 1 drop).

Consecutive titrations should agree to within 0.10 cm3 and, strictly, you should repeat the titrations until this is achieved. However you may not have either the time or materials available to do this.

With practice, your technique should improve so that you should not need to do more than 4 titrations (1 trial + 3 accurate).

Calculate and use the mean (average) of the two (or preferably three) closest consecutive readings and quote this to the nearest 0.05 cm3.

 

Calculating the Concentration of a Solution from Titration Data

 

When you have finished this section you should be able to:

·         Calculate the concentration of a solution from titration data and the balanced equation.

·         Calculate the volume of solution required for titration from titration data and the balanced equation

 

Volumetric calculations are a little more complicated than those you will have done previously, because they involve several steps.  Each step is very simple, but you may not be able to see immediately where to start.  Before we look at titration calculations we will look at a general approach to solving multi-step problems which may be useful to you in more difficult problems.

 

An approach to solving multi-step problems

Ask yourself three questions.

 

1.        What do I know?

2.       What can I get from what I know?

3.       Can I now see how to get the final answer?

In most cases the answer to question 3 will be ‘Yes’, but you may need to ask question 2 again, now that you know more than you did at the start.

 

Example 1

25.0 cm3 of a solution of barium hydroxide, Ba(OH)2, of unknown concentration is placed in a conical flask and titrated with a solution of hydrochloric acid, HCl, which has a concentration of 0.0600 mol dm-3.  The volume of acid required is 20.40 cm3.  Calculate the concentration of the barium hydroxide solution.

Ba(OH)2 (aq)  +  2HCl (aq)                                 BaCl2 (aq)  +  2H2O (l)
 
Solution
A summarised flow-chart for the solution of this problem is:

 

EQUATION                              relative amounts

 

VOLUME HCl

                                                Amount HCl                   amount Ba(OH)2            conc.

CONCENTRATION HCl                                                                         Ba(OH)2

 

VOLUME Ba(OH)2

Now we look at the calculation step by step.

 

1.        Calculate the amount of HCl delivered (the solution of known concentration) by substituting in the expression

c = n/V in the form  n = cV

where c = 0.0600 mol dm-3 and V = (20.40/1000) dm3 = 0.0204  dm3

n = cV = 0.0600 mol dm-3 x  0.0204 dm3 =  1.22 x 10-3 mol. Calculate the amount of Ba(OH)2 (the solution of unknown concentration) which reacts with this amount of HCl by substituting in the expression derived from the equation.

 

Amount of Ba(OH)2   =   ˝

  Amount of HCl

Amount of Ba(OH)2 = ˝ x  amount of HCl

                              =  ˝  x  1.22  x  10-3 mol

                              =  6.12 x 10-4 mol

 

2.       Calculate the concentration of Ba(OH)2 by substituting into the expression

c = n/V

where n = 6.12 x 10-4 mol  and  V = 0.0250 dm3  =  2.50 x 10-2 dm3

c = n/V  =  6.12 x 10-4 mol   =  6.12 x 10-2 mol         =    2.45 x 10-2 mol dm-3    

2.50 x 10-2 dm3      2.50 dm3

[0.0245 mol dm-3]

 

 

A General Expression

Every titration problem relies on five pieces of information, one of which is unknown:

·         A balanced chemical equation.

·         The concentration M1 of the first reagent.

·         The reacting volume V1 of the first reagent.

·         The concentration M2 of the second reagent.

·         The reacting volume V2 of the second reagent.

 

Consider the general equation:

                        n1 A  +  n2 B                               Products

It is possible to derive an expression relating the concentration of solution A (M1), reacting volume of solution A (V1), concentration of solution B (M2), reacting volume of solution B (V2) and the coefficients n1 and n2 from the equation.

 

From the chemical equation we obtain the relationship:

                        Amount of A     =          n1

                        Amount of B                  n2

We know that :

                        Amount of A     =  M1V1

            and       Amount of B      =  M2V2

 

Substituting for the amount of A and the amount of B in the first expression gives:

                        M1V1   =   n1

                M2V2       n2

 

This expression is very useful for titrimetric problems and can be used when appropriate. However it may not always provide the best way to tackle a particular problem.

 

 

Exercise 1

(a)    Solve Example 1 above using the expression derived.  [0.0375 mol dm-3]

(b)    Why is it not necessary to convert volumes in cm3 to dm3 when using the expression?  [It is not necessary to convert from cm3 to dm3 because the units of volume cancel in the final expression]

(c)    In a titration, 25.0 cm3 of 0.100 mol dm-3 sodium hydroxide were found to react exactly with 11.2 cm3 of sulphuric acid.

Find the concentration of the sulphuric acid.          [0.112 mol dm-3]

 

Exercise 2

What volume of sodium hydroxide solution, 0.500 M NaOH is needed to neutralise

(a)    50.0 cm3 of nitric acid, 0.100 M HNO3,

NaOH (aq)  +  HNO3 (aq)                            NaNO3 (aq)  +  H2O (l)

[10.0 cm3]

 

(b)    22.5 cm3 of sulphuric acid,0.282 M H2SO4,

2NaOH (aq)  + H2SO4 (aq)                          Na2SO4 (aq)  +  2H2O (l)

[23.6 cm3]

 

 

 

Standard solutions

 

When you have finished this section you should be able to:

·         Define concentration.

·         Explain the term standard solution.

·         Calculate the concentration of a solution given the amount or mass of solute and the volume of solution.

 

 

A standard solution is one of known concentration.  We can either make up a solution of known mass and volume or analyse it to determine its concentration.

The letter M is sometimes used as an abbreviation for mol dm-3 but should only be used with a formula.  For example if 0.0100 mol of sodium hydroxide are dissolved and made up accurately to 1 dm3 of solution its concentration can be written as 0.0100 M NaOH.

 

 

Exercise 3

Calculate the molarity of

(a)    23 g of ethanol C2H5OH in 1 dm3 of solution.    [0.500 M]

(b)    Hydrogen ions in 1 dm3 of hydrochloric acid solution containing 3.65 g of hydrogen chloride (assume the acid is fully ionised).  [0.100 M]

(c)    Hydroxide ions in 1 dm3 of a solution containing 17.1 g of barium hydroxide.  [0.200 M]

(d)    Sulphate ion in a solution of aluminium sulphate Al2(SO4)3.12H2O of concentration 0.100 mol dm-3.  [0.300 M]

(e)    Aluminium ion in a solution of aluminium sulphate Al2(SO4)3.12H2O of concentration 0.100 mol dm-3.  [0.200 M]

 

Acid-base Titrations

Acid-base titrations involving strong acid/strong base, strong acid/weak base and weak acid/ strong base, e.g. determination of the degree of hydration in a sample of sodium carbonate, analysis of vinegar. The knowledge of suitable indicators for these titrations.

 

When you have finished this section you should be able to:

·         Prepare a standard solution

·         Perform titrations

·         Use titration data to determine the concentration of a solution

·         Determine the number of molecules of water of crystallisation from titration data

 

 

ADVANCED LEVEL CHEMISTRY PRACTICAL ASSESSMENT

 

Candidate Number

 

Centre Number

71637

 

Name

 

Date

 

 

EXPERIMENT 1

Preparation of a standard solution

 

SKILL AREAS ASSESSED :   1  Manipulation, measuring & recording

3  Concluding & Communicating (Quantitative)

AIM

The purpose of this experiment is to prepare a standard solution of potassium hydrogenphthalate.

INTRODUCTION

You weigh accurately a sample of potassium hydrogenphthalate, C8H5O4K, and use it to make a solution of concentration close to 0.100 mol dm-3. Later you will use this solution to determine the concentration of a solution of sodium hydroxide.

 

PROCEDURE

1.        Transfer between 4.8 g and 5.4 g of potassium hydrogenphthalate into a weighing bottle and weigh it to the nearest 0.01 g.

2.       Put about 50 cm3 of water into a 250 cm3 beaker. Carefully transfer the bulk of the potassium hydrogenphthalate from the weighing bottle into the beaker.

3.       Reweigh the bottle with any remaining potassium hydrogenphthalate residue to the nearest 0.01 g.

4.       Stir to dissolve the solid, adding more water if necessary.

5.       Transfer the solution to the 250 cm3 volumetric flask through the filter funnel. Rinse the beaker well, making sure all the liquid goes into the volumetric flask. (Some workers transfer the solid directly into the flask through a filter funnel, but you should only do this if you are sure the solid will dissolve easily and if the funnel has a wide enough stem to prevent blockage.)

6.       Add distilled water, swirling at intervals to mix the contents, until the level is within 1 cm of the mark on the neck of the flask.

7.       Using a dropping pipette, add enough water to bring the bottom of the meniscus to the mark. Insert the stopper and shake thoroughly ten times to ensure complete mixing. (Simply inverting the flask once or twice does not mix the contents properly and is a very common fault.)

8.       Label the flask with the contents, your name and date. Leave a space for the concentration to be filled in after you have calculated it. Set the flask aside for use later.

 

CONCLUDING & COMMUNICATING

1.        Complete the results table below.

 

Molar mass of potassium hydrogenphthalate

 

Mass of bottle and contents before transfer

 

Mass of bottle and residue after transfer</