Volumetric Analysis

Percentage by Weight

The mass percentage of a component in a given solution is the mass of the component per 100g of the solution. For e.g.If WA is the mass of the component A, WB is the mass of the component B in a solution. Then,

Mass percentage of A in a solution =WAWA+WB * 100

Example: A 20% solution of sodium chloride in water (by mass) means that 20g of sodium chloride are present in 100g of the solution.

Percentage by Volume

This unit is used in case of a liquid dissolved in another liquid. The volume percentage is defined as the volume of the solute per 100 parts by volume of solution.

For e.g., If VA is the volume of component A present is Vs volume of the solution.

Then,

Volume percentage of A in the solution =VAVs * 100

For e.g., a 20% solution of ethanol C2H5OH, in water (by volume) means that 20cm3 of ethanol is present in 100cm3 of the solution.

Strength of a solution is defined as the amount of the solute in gm, present in one liter of the solution. It is expressed as gL-1.

Mathematically,

Strength =MassofsoluteingVolumeofsolninlitres

Molarity:

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

Mathematically,

Molarity=NoofmolesofsoluteVolumeofsolutioninlitres

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

Then,

Molarity =mmolmass$\frac{\mathrm{a}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}}$ * 1000V$\frac{1000}{\mathrm{V}}$

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

Molarity =Strengthingmsperlitremolecularmassofthesolute

Normality:

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 =Numberofeq.ofsoluteVolumeofsolutioninlitres

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

Normality =meq.mass$\frac{\mathrm{a}}{\mathrm{e}\mathrm{q}.\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}}$ *1000V$\frac{1000}{\mathrm{V}}$

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

Normality =strengthingmsperlitreEqmassofthesolute

 Relationship between molarity and normality

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

Normality = Molarity *MolecularmassofsoluteEq.massofsolute

Simply,

N=nM

Molality:

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

Molarity = NumberofmolessoluteEq.massofsolute

Molality is expressed by the symbol m

note:Molarity with M and molality with m

Mole Fraction:

It is the ratio of number of moles of one component (solute or solvent) to the total number of moles of all the components (solute and solvent) present in the solution. It is denoted by the symbol X. Let us suppose that a solution contains two components A and B and suppose that nA moles of A and nBmoles of B are present in the solution then,

Mole fraction of A


 XA =nAnA+nB…….. (i)

Mole fraction of B XB =nAnA+nB…….. (ii)

  

Titration:

Selection of acid base titration:

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

- Titration involving strong acid and strong base:

In this type of reaction, there is a sharp change in pH around neutralization point. The pH of this point varies from 3.3 to 10.7. Thus, a indicator of range 3 – 10.5 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 on the pH range of 3.5 to 7.0. So, acidic indicator can be used like methyl orange.

- Titration involving strong base and weak acid:

The titration has equivalent point in the pH range of 7.7 to 9.7. Hence, basic indicator like phenolphthalein may be used.

-Titration involving weak acids and weak base.

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

Redox titration:

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

Gram equivalent weight:It is defined as the no. of parts by weight in gm of a chemical substance which combines with or is displaced by 1.008gm of Hydrogen or its equivalent, i.e. 8gm of oxygen and 35.5 gm of chlorine.

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

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

Primary standard substance: The substance whose standard solution can be prepared directly by weight is known as primary, standard substance.

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

Primary standard solution: The solution whose standard solution can be prepared by directly dissolving required amount of solute in it is called primary standard solution.

 

Indicator: An indicator is the reagent used in titration to detect the end point i.e. completion of the reaction.

 

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.

The normality equation is commonly used to calculate the normality of solutions after dilution. The above equation is also called as dilution formula because it helps in calculating the volume of the solvent required in diluting a concentrated solution. The same equation holds good for calculations involving molarity (M).

we can also write,

V1S1=V2S2

where,

V=volume of the given substance

and

S=Strength of given substance