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Unit 2Unit 2Stoichiometry – Volumetric Stoichiometry – Volumetric

analysisanalysis

Stoichiometry Stoichiometry

Any reaction in which the substances Any reaction in which the substances react completely according to the react completely according to the

mole mole ratios given by a balanced ratios given by a balanced (stoichiometric) equation is called a (stoichiometric) equation is called a quantitative reaction.quantitative reaction. When a quantitative reaction takes When a quantitative reaction takes place, place, if the quantity of one substance if the quantity of one substance is known is known then another the unknown then another the unknown quantity of quantity of another can be another can be determined.determined.

Two chemical methods of Two chemical methods of quantitative analysis arequantitative analysis are

1.1. Volumetric analysisVolumetric analysis

2.2. Gravimetric analysisGravimetric analysis

Volumetric analysisVolumetric analysis

Volumetric analysis involves using a Volumetric analysis involves using a solution of accurately known solution of accurately known concentration to determine the concentration to determine the concentration of the other solution concentration of the other solution involved in the reaction. This is involved in the reaction. This is called a titration.called a titration.

There are three main types of titration:There are three main types of titration:1.1. acid/baseacid/base2.2. complexometriccomplexometric3.3. redoxredox

To carry out a titration, one solution To carry out a titration, one solution must be pre-prepared accurately using must be pre-prepared accurately using a standard flask and all volumes must a standard flask and all volumes must be measured using a pipette or burettebe measured using a pipette or burette

Standard solutionsStandard solutions The pre-prepared solution of accurately The pre-prepared solution of accurately

known concentration is known as a known concentration is known as a standard solution.standard solution.

A standard solution can only be A standard solution can only be prepared from a primary standard prepared from a primary standard which must be readily available and which must be readily available and have the following characteristics:have the following characteristics:

- a high purity- a high purity- be stable in air and in solution- be stable in air and in solution- a reasonably high formula mass- a reasonably high formula mass- be readily soluble (usually in - be readily soluble (usually in

water)water)

The first three characteristics The first three characteristics ensure that the sample is weighed ensure that the sample is weighed out accurately.out accurately.

The most important characteristic The most important characteristic is the primary standard’s stability is the primary standard’s stability in air e.g. NaOH cannot be used in air e.g. NaOH cannot be used because it absorbs both moisture because it absorbs both moisture and COand CO22 from the air. from the air.

Preparing standard Preparing standard solutionssolutions

Calculate the mass of solute required to Calculate the mass of solute required to make concentration of solution required.make concentration of solution required.

Dissolve in small amount of deionised Dissolve in small amount of deionised water.water.

Pour into standard flask through funnel.Pour into standard flask through funnel. Wash beaker into standard flask using Wash beaker into standard flask using

wash bottle.wash bottle. Make up to the line with deionised water.Make up to the line with deionised water. Mix thoroughly by inverting flask.Mix thoroughly by inverting flask.

Common primary Common primary standardsstandards

Acid/base titrations – oxalic acid Acid/base titrations – oxalic acid and anhydrous (dry) sodium and anhydrous (dry) sodium carbonatecarbonate

Complexometric – EDTAComplexometric – EDTA

Redox – potassium iodate.Redox – potassium iodate.

Equivalence pointEquivalence point Once a standard solution has been Once a standard solution has been

prepared it is titrated with a known prepared it is titrated with a known volume of solution of unknown volume of solution of unknown concentration.concentration.

The point at which the reaction is just The point at which the reaction is just complete is called the equivalence point.complete is called the equivalence point.

An equivalence point must be observed An equivalence point must be observed either by a colour change in the reaction either by a colour change in the reaction or by adding an indicator that changes or by adding an indicator that changes colour at the equivalence point.colour at the equivalence point.

The colour change occurs when the end-The colour change occurs when the end-point of the titration has been reached.point of the titration has been reached.

End Point of TitrationEnd Point of Titration

Acid/base : an indicator with a Acid/base : an indicator with a relevant pH range must be used to relevant pH range must be used to determine the end-point of the determine the end-point of the reaction e.g. phenolphthalein or reaction e.g. phenolphthalein or methyl orange.methyl orange.

Complexometric : the colour changes at the end-Complexometric : the colour changes at the end-point due to the formation of a transition metal point due to the formation of a transition metal complex. EDTA complexes with metal ions in a complex. EDTA complexes with metal ions in a one to one ratio.one to one ratio.

The indicator has to complex with the metal ion The indicator has to complex with the metal ion to give a visible colour that is different to the to give a visible colour that is different to the uncomplexed indicator. The indicator must also uncomplexed indicator. The indicator must also bond to the metal ion less well than EDTA so bond to the metal ion less well than EDTA so that when EDTA is added the indicator is that when EDTA is added the indicator is displaced.displaced.

A colour change is observed when all the A colour change is observed when all the indicator is displaced.indicator is displaced.

Murexide is an excellent indicator for calcium Murexide is an excellent indicator for calcium and nickel ions.and nickel ions.

Redox : potassium permanganate [potassium Redox : potassium permanganate [potassium manganate (VIII)] is used as it acts as its own manganate (VIII)] is used as it acts as its own indicator.indicator.

It is decolourised in a redox reaction and It is decolourised in a redox reaction and therefore, the end-point occurs when a very therefore, the end-point occurs when a very pale pink colour due to an excess of pale pink colour due to an excess of manganate (VII) is observed.manganate (VII) is observed.

One problem with this indicator is that the One problem with this indicator is that the bottom of the meniscus in the burette is bottom of the meniscus in the burette is difficult to read because of the very dark difficult to read because of the very dark colour of the solution. This is overcome by colour of the solution. This is overcome by reading the scale at the top of the meniscus.reading the scale at the top of the meniscus.

TitrationsTitrations

Set up burette vertically and rinse Set up burette vertically and rinse with standard solution. Fill and with standard solution. Fill and record the top reading.record the top reading.

Rinse the pipette with the solution Rinse the pipette with the solution of unknown concentration. Fill to of unknown concentration. Fill to the graduation mark and add a the graduation mark and add a known volume of the unknown to a known volume of the unknown to a conical flask.conical flask.

Add a few drops of indicator to the Add a few drops of indicator to the conical flask.conical flask.

Place the conical flask on a white tile to Place the conical flask on a white tile to make the end-point easier to see.make the end-point easier to see.

Run the standard solution from the Run the standard solution from the burette into the unknown while burette into the unknown while continually swirling the flask.continually swirling the flask.

The burette jet must be just inside the The burette jet must be just inside the flask to avoid missing the flask during flask to avoid missing the flask during swirling.swirling.

As the end-point is approached the As the end-point is approached the solution is added drop-wise until solution is added drop-wise until the indicator shows a permanent the indicator shows a permanent colour change.colour change.

The reading on the burette is The reading on the burette is recorded again.recorded again.

The titre is the second reading The titre is the second reading minus the first reading.minus the first reading.

Normally one rough titration and Normally one rough titration and two accurate titrations (within two accurate titrations (within 0.1cm0.1cm33 of each other) are taken. of each other) are taken.

The two accurate titrations are The two accurate titrations are then averaged to give the value then averaged to give the value used in the calculation to find the used in the calculation to find the concentration of the unknown.concentration of the unknown.

Volumetric Calculations Volumetric Calculations

Example Example

10.0 cm10.0 cm33 of lithium hydroxide solution of lithium hydroxide solution was neutralised by 16.7 cmwas neutralised by 16.7 cm33 of 0.1 mol l of 0.1 mol l--

11 phosphoric acid solution. phosphoric acid solution.

Calculate the concentration of the Calculate the concentration of the lithium hydroxide solution.lithium hydroxide solution.

3LiOH + H3LiOH + H33POPO44 →→ Li Li33POPO44 + 3H + 3H22OO

Using the equation:Using the equation:

ccaa x v x vaa = = ccbb x v x vbb

a ba b

0.1 x 0.01670.1 x 0.0167 = = CCbb x 0.010 x 0.010

1 31 3

Cb = Cb = 3 x 0.00167 3 x 0.00167 = = 0.501mol l0.501mol l-1-1

0.0100.010

ExerciseExercise

Now try the exercise on page 6 of Now try the exercise on page 6 of the Unit 2(a) booklet.the Unit 2(a) booklet.

You may have to revise adding ion-You may have to revise adding ion-electron equations together from electron equations together from the Higher course first.the Higher course first.