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Volumetric Analysis

110

VOLUMETRIC ANALYSIS


Molarity:

The molarity of a solution is defined as the number of moles of solute dissolved per litre of solution.

Mathematically,

 Molarity = No of moles of solute/volume of solution in litres

 For e.g If 'X' is the weight of the solute (in gms) present in VCC volume of the solution.

Then,

Molarity = (X/molar mass) * (1000/V)

Molarity is expressed by the symbol M. It can also be expressed as,

 Molarity = Strengthingmsperlitre / Molecular mass of the solute


Normality:

The normality of a solution is defined as the number of gram equivalents of a solute dissolved per liter of the given solution.

Mathematically it is,

 Normality = Number of eq. wt of solute/volume of solution in litres

 For e.g., If 'X' is the weight of the solute (in gm) present in VCC volume of the solution. Then,

Normality =( X/eq mass) * (1000/V)

Normality is expressed by the symbol N. It can also be expressed as,

 Normality = Strength in gms per litre / Eq Mass of the solute


Molality:

The molality of a solution is defined as the number of moles of solute dissolved in 1000g of a solvent. Mathematically, it is expressed as,

 Molality = (No of gram moles of solute/weight of solvent in g) *1000

 Molality is expressed by the symbol m or molal.

Molality does not change with temperature.


Gram/litre:

It indicates the amount of solute in gram present in one litre (1000ml) of solution.

Mathematically,

Gm/Lit = weight of solute(gm) / volume of solution(lit.)

            = (weight of solute(gm) / volume in solution(ml)) * 1000


Percentage:

(i) Percentage by volume (%w/v)

It indicates the weight of solute in gram present in 100ml of solution.

Mathematically,

%w/v = (weight of solute(gm)/volm of soln(ml))  *100

(ii) Percentage by weight (%w/w)

It indicates the amount of solute in gram present in 100gm solution.

%w/w = (weight of solute(gm)/volm of soln(gm)) * 100


Relationship between molarity and normality

The molarity and normality of a solution are related to each other as follows:

Normality = Molarity * (Molecular mass of solute/eq mass of solute)


Derivation of normality equation:

To calculate the volume of a definite solution required to prepare solutions of other normality, the following equation is used:

 N1V1=N2V2

 Where,

N1=initial normality and N2=normality of the new solution,

and V1= initial volume and V2= volume of the new solution.


Primary standard substance: The substance whose standard solution can be prepared directly by weight is known as a primary standard substance. Features of a primary standard include:

  • High purity.
  • Stability (low reactivity)
  • Low hygroscopicity (to minimize weight changes due to humidity)
  • High equivalent weight (to minimize weighing errors)
  • Non-toxicity.
  • Ready and cheap availability.

 

Secondary standard solution: The substance whose standard solution can’t be prepared directly by weighing but by titration with primary standard solution are the secondary standard solution.

Equivalence point: It is defined as the theoretical point in titration equivalents of acid neutralized equivalents of the base to bring complete neutralization.

Endpoint: It is defined as the point in titration, as observed by a sharp change in color of indicator due to neutralization.


Titration:

Redox titration:

Redox titration (also called oxidation-reduction titration) is a type of titration based on a redox reaction between the analyte and titrant. Redox titration may involve the use of a redox indicator and/or a potentiometer.

Selection of acid-base titration:

The selection of indicators is an important part of volumetric analysis. An acid-base titration is usually of four types and each sets definite criteria for the selection of indicators.


-Titration involving strong acid and strong base:

In this type of reaction, there is a sharp change in pH around the neutralization point. The pH of this point varies from 3 to 11. Thus, an indicator of range 3 – 11 can be used. Hence both phenolphthalein and methyl orange can be used.

 


-Titration involving strong acid and weak base:

In this type of titration the equivalent point lies in the pH range of 3 to 8. So, the acidic indicators can be used like methyl orange.

 

- Titration involving strong base and weak acid:

The titration has equivalent point in the pH range of 6 to 9. Hence, basic indicators like phenolphthalein may be used.

 

-Titration involving weak acids and weak bases.

Since there is no sharp change in pH, accurate measurement of endpoint, in this case, is not shown by indicators.