Volumetric AnalysisVolumetric Analysis

Chapter 3Chapter 3

ConcentrationConcentration

What equation do we know involving What equation do we know involving concentration?concentration?

n = cVn = cV Concentration is expressed in mol LConcentration is expressed in mol L-1-1, ,

which is given the symbol Mwhich is given the symbol M Another measure for concentration is mass Another measure for concentration is mass

per unit volume, g Lper unit volume, g L-1-1, this is found easily , this is found easily by dividing the mass (in g) by the volume by dividing the mass (in g) by the volume (in L).(in L).

To convert mol LTo convert mol L-1-1 to g L to g L-1-1 multiple the multiple the molarity by the molar mass of the solutemolarity by the molar mass of the solute

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Page 34Page 34 Question 11aQuestion 11a Question 13Question 13 Question 14b,cQuestion 14b,c

Standard SolutionStandard Solution

A solution with an accurately known A solution with an accurately known concentration is called a standard solution.concentration is called a standard solution.

You can not simply make a standard You can not simply make a standard solution by dissolving a measured mass of solution by dissolving a measured mass of a substance in water to give a known a substance in water to give a known volume of solution. It just doesn’t always volume of solution. It just doesn’t always work. work.

Only a few chemicals encountered can do Only a few chemicals encountered can do this, many chemicals are impure as they this, many chemicals are impure as they react with the atmosphere or decomposereact with the atmosphere or decompose

Standard SolutionsStandard Solutions

Substances that are so pure that the Substances that are so pure that the amount of substance, in mole, can be amount of substance, in mole, can be calculated accurately from their mass are calculated accurately from their mass are called primary standards.called primary standards.

A primary standard should:A primary standard should: Be readily obtainable in pure formBe readily obtainable in pure form Have a known formulaHave a known formula Be easy to store without deteriorating or Be easy to store without deteriorating or

reacting with the atmospherereacting with the atmosphere Have a high molecular mass to minimise the Have a high molecular mass to minimise the

effect of errors with weighingeffect of errors with weighing Be inexpensiveBe inexpensive

Standard SolutionsStandard Solutions Page 29 look at some examples of Page 29 look at some examples of

primary standards.primary standards.

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Page 34Page 34 Question 12aQuestion 12a

Volumetric AnalysisVolumetric Analysis

Helps us to find the amount of a substance Helps us to find the amount of a substance in solutionin solution

Volumetric analysis involves reacting a Volumetric analysis involves reacting a measured volume of a standard solution measured volume of a standard solution with a measured volume of the solution of with a measured volume of the solution of unknown concentrationunknown concentration

Ideally the solutions are mixed until they Ideally the solutions are mixed until they have just reacted completely in the mole have just reacted completely in the mole ratio indicated by the stoichiometric ratio indicated by the stoichiometric equation.equation.

What is this process called????What is this process called????

Volumetric AnalysisVolumetric Analysis

Turn to page 30 and 31Turn to page 30 and 31

Equivalence pointEquivalence point

The equivalence point is the point The equivalence point is the point during the titration when the solutions during the titration when the solutions have been mixed in the mole ratio have been mixed in the mole ratio shown by the reaction equation. shown by the reaction equation.

ExampleExample2HCl(aq) + Ca(OH)2HCl(aq) + Ca(OH)22(aq) (aq) → CaCl→ CaCl22(aq) + H(aq) + H22O(l)O(l)

The equivalence point is reached when the mole The equivalence point is reached when the mole ratio n(HCl) : n(ratio n(HCl) : n(Ca(OH)Ca(OH)22) = 2:1) = 2:1

So So n(HCl) = 2n(n(HCl) = 2n(Ca(OH)Ca(OH)22))

Equivalence point and end pointEquivalence point and end point

An indicator must be used in acid-An indicator must be used in acid-base titrations to detect the base titrations to detect the equivalence point.equivalence point.

The change in colour during a The change in colour during a titration is the end pointtitration is the end point

So you need an indicator with an end So you need an indicator with an end point which closely matches the point which closely matches the equivalence pointequivalence point

AccuracyAccuracy

There are always errors associated There are always errors associated with measurements made during with measurements made during experimental work. experimental work.

Typical undertainties associated with Typical undertainties associated with volumetric analysis are:volumetric analysis are: 20 mL pipette 20 mL pipette ±0.05 mL±0.05 mL Burette ±0.02 mL for each readingBurette ±0.02 mL for each reading 250.0 mL volumetric flask ±0.3 mL250.0 mL volumetric flask ±0.3 mL

Accuracy in titrationsAccuracy in titrations

We always do 3 concordant We always do 3 concordant titres. Why?titres. Why?

This is because the volume This is because the volume of a single drop from a of a single drop from a burette is about 0.05 mLburette is about 0.05 mL

So each concordant titre is So each concordant titre is within a drop of each other.within a drop of each other.

Remember we always use Remember we always use the average titrethe average titre

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Page 34 Page 34 Question 23Question 23

For homeworkFor homework

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Question 16 and 21Question 16 and 21