Volumetric analysis

Equivalent Weight

Equivalent wt of an element is defined as the no. of parts by wt of that element which can combine or displaced directly or indirectly 1.008 parts by wt of hydrogen, 8 parts by wt of oxygen or 35.5 parts by wt of chlorine.

Example: CH₄, MgO, AlCl₃

1) CH₄

1.008 × 4 parts by wt of hydrogen combined with 12 parts by wt of carbon.

1 parts by wt of hydrogen combine with 12 / (1.008 × 4) parts by wt of carbon.

1.008 parts by wt of hydrogen combines with:

Eq wt of C in CH₄ is 3.

2) MgO

16 parts by wt of oxygen combine with 24 parts by wt of magnesium.

1 parts by wt of oxygen combines with 24/16 parts by wt of magnesium.

8 parts by wt of oxygen combines with:

Eq wt of Mg in MgO is 12.

3) AlCl₃

35.5 × 3 parts by wt of chlorine combine with 27 parts by wt of Al.

1 parts by wt of chlorine combine with 27 / (35.5 × 3) parts by wt of Al.

35.5 parts by wt of chlorine combine with:

For an element

eg: C, Ca, Al, Mg

For compound

where, Z = basicity for acid. [Total no. of replaceable H⁺ ion]

Example: Acid

Example Base

Example: Salt

For radical

eg CO₃²⁻

SO₄²⁻

PO₄³⁻

NH₄⁺

NO₃⁻

Cl⁻

Ca²⁺

For oxidising Reducing agent

eg KMnO₄ (potassium permanganate)

a) In acidic medium

change in ON of Mn = 5

eq wt in acidic medium = 31.6

b) In basic medium

change in ON of Mn = 1

eq wt in basic medium = 158

c) In neutral medium

change in ON of Mn = 3

Volumetric Analysis

Gram equivalent wt

When equivalent wt of a substance is expressed in gram which is known gram equivalent wt.

NOTE: No of gram equivalent wt of any substance = no of eq wt of that substance.

1g eq wt of Na₂CO₃ = Eq wt of Na₂CO₃ = 53g of Na₂CO₃.

1g eq wt of H₂SO₄ = Eq wt of H₂SO₄ = 49g of H₂SO₄.

Concentration / strength of soln

1. Gram per litre (g/L)

Gram per litre of a substance define as a weight of substance in gram present in 1 litre of solution. It is denoted by (g/L) and given by:

2. Normality (N)

Normality of a soln is define as the no of gram equivalent wt of substance present in 1 litre of solution. It is denoted by N and given by:

Again:

a) Normal solution (1N or N)

When 1g eq wt of any substance is present in 1 litre of solution then soln is called normal solution. It is denoted by 1N or N.

b) Semi-Normal solution (N/2 = 0.5N)

When half g eq wt of substance is present in 1 litre of solution then soln is called semi-solution. It is denoted by N/2 or 0.5N.

c) Deci-Normal soln (N/10 = 0.1N)

When 1/10 g eq wt of substance is present in 1 litre of solution then soln is called deci-normal soln. It is denoted by N/10 or 0.1N.

d) Centi-Normal soln (N/100 = 0.01N)

When 1/100 g eq wt of substance is present in 1 litre of solution then soln is called centi-normal soln. It is denoted by N/100 or 0.01N.

Molarity (M)

Molarity of a solution is define as the no of mole of substance present in 1 litre of solution. It is denoted by M and given by:

Again:

a. Molar soln (1M = M)

When 1 mole of substance is present in 1 litre of solution then the solution is called molar soln. It is denoted by 1M or M.

b. Semi-molar soln (M/2 = 0.5M)

When half mole of substance is present in 1 litre of solution then the soln is called semi-molar soln. It is denoted by M/2 or 0.5M.

c. DECI-Normal solution (M/10 or 0.01M)

When 1/10 mole of substance is present in 1 litre of solution then the soln is called deci-normal solution.

d. Centi-molar solution (M/100 or 0.01M)

When 1/100 mole of substance is present in 1 litre of soln then the soln is called centi molar soln. It is denoted by M/100 or 0.01M.

Relation bet’n normality and molarity

We have:

For an acid:

For a base:

For a salt:

For an oxidising and reducing agent:

Relation of % with normality and molarity

For normality:

If specific gravity (density) is given:

For molarity:

If specific gravity (density) is given:

4. Molality (m)

molality of a soln is define as the no of mole of substance present in 1 kg of solvent. It is denoted by m and given by:

a) Molal soln

When 1 mole of substance is present in 1kg of solvent then the soln is called molal soln. It is denoted by 1m or m.

b) Semi molal soln (m/2 = 0.5m)

When 1/2 mole of substance is present in 1kg of solvent then the soln is called semimolal soln. It is denoted by m/2 or 0.5m.

c) Deci-molal soln (m/10 or 0.1m)

When 1/10 mole of substance is present in 1kg of solvent then the soln is called deci-molal soln. It is denoted by m/10 or 0.1m.

d) Centi-molal soln (m/100 or 0.01m)

When 1/100 mole of substance is present in 1kg of solvent then the soln is called centi-molal soln. It is denoted by m/100 or 0.01m.

5. Formality (F)

Formality of a solution is defined as the no of gram of formula wt present in 1 litre of soln and it is denoted by (F) and given by:

Again:

i) Formal soln (1F or F)

When 1g formula wt of substance is present in 1 litre of soln is called formal soln. It is denoted by 1F or F.

ii) Semi-formal soln (F/2 = 0.5F)

When 1/2 g formula wt of substance present in 1 litre soln then the soln is called semi-formal soln. It is denoted by F/2 or 0.5F.

iii) Deci-formal soln (F/10 or 0.1)

When 1/10 g formula wt of substance present in 1 litre of soln then the soln is called deci-formal soln. It is denoted by F/10 or 0.1.

iv) Centi-formal soln (F/100 or 0.01)

When 1/100 g formula wt of substance present in 1 litre of soln then the soln is called centi-formal soln. It is denoted by F/100 or 0.01.

6) Percentage concn

percentage concn of a soln is defined as the wt of substance in g or ml present in 100g or 100 ml of solution. It is of following type.

1) wt by wt percentage [(w/w) × %]

Wt by wt percentage of a soln is defined as the wt of substance in gram present in 100 gram of solution. It is denoted by (w/w)% and given by:

eg: 1% (w/w) NaOH means:

1g of NaOH is present in 100g of solution.

eg: 0.5% (w/w) KOH means:

0.5g of KOH is present in 100g of solution.

2. Wt by volume percentage (w/v) × %

Wt by volume percentage of a soln is defined as the wt of substance in gram present in 100 ml of soln. It is denoted by w/v and given by:

Eg: 1% w/v sodium carbonate (Na₂CO₃) means:

1 gram of Na₂CO₃ is present in 100 ml of soln.

Eg: 0.1% w/v oxalic acid [(COOH)₂] means:

0.1 gram of (COOH)₂ is present in 100 ml of soln.

3. Volume by volume percentage (v/v)%

Volume by volume percentage of a soln is defined as the wt of substance in ml present in 100 ml of soln. It is denoted by v/v and given by:

eg 1% (v/v) H₂SO₄ means:

1 ml of H₂SO₄ is present in 100 ml of soln.

7) Parts per million (ppm)

parts per million soln is define as the wt of substance in gram present in 1 million parts by wt of soln and given by:

8) Parts per billion (ppb)

parts per billion soln is define as the wt of substance in gram present in 1 billion parts by wt of soln. It is denoted by ppb and given by:

9) Mole fraction

Mole fraction of soln is define as the ratio of no of mole of 1 component to the no of mole of all the component (solute and solvent).

let w consider n₁ be the no of mole of solute and n₂ be the no of mole of solvent then total no of mole of soln is (n₁ + n₂).

Mole fraction of solute + mole fraction of solvent:

Imp Primary and secondary standard substances

The exact wt of some substance cannot be taken because they may be volatile, hygroscopic, deliquescent, efflorescent, [Unreadable in source] and composition is different in soln and solid state. When they are dissolved in fixed volume of soln then its exact concn is not known is called secondary substances and corresponding solution are called secondary standard solutions. eg sodium hydroxide, hydrochloric acid, sulphuric acid, nitric acid, potassium hydroxide, potassium permanganate (KMnO₄).

When exact wt of some substances can be taken and its concn is known by dissolving in fixed volume of soln is called primary standard substances and corresponding soln are called primary standard soln.

eg: oxalic acid, sodium carbonate (Na₂CO₃), copper sulphate, sodium chloride (NaCl), potassium chloride, potassium dichromate (K₂Cr₂O₇), mohr’s salt [FeSO₄·(NH₄)₂SO₄·6H₂O].

Condition to be of primary standard substances

• The substance should be pure, dry and soluble in water.
• The substance should have high molecular wt and equivalent wt.
• The substance should not be volatile, hygroscopic, deliquescent, [Unreadable in source] & efflorescent.
• The substance should be stable (i.e) its composition should not change in soln or in solid for long time.

Titration

The process of determining the concn of unknown soln by gradual addition of one soln in the other solution in presence of third chemical substance i.e indicator is called titration. It is of following types.

1) Acid base titration

The process in which the concn of an acid is determine with the help of standard base (alkali soln) in presence of indicator or vice versa is called acid base titration.

Eg: titration betn H₂SO₄ and Na₂CO₃ – NaOH / (COOH)₂.

i) Acidimetry

The process of determining concn of an acid by finding the exact volume of it by neutralizing with known volume of standard base (alkali) in presence of suitable indicator is called acidimetry.

ii) Alkalimetry

The process of determining concn of a base by finding the exact volume of it by neutralizing with known volume of standard acid in presence of suitable indicator is called alkalimetry.

2) Redox titration

The titration in which the concn of oxidising agent is determine with the help of primary standard reducing agent or vice versa is called redox titration.

Eg: titration bet’n KMnO₄ and oxalic acid soln in presence of sulphuric acid (H₂SO₄).

Imp: KMnO₄ is an oxidizing agent and oxalic acid is reducing agent whereas KMnO₄ act as self indicator.

Different terms used in titration

i) Titrant

The volm of soln which is taken in a burette of known concn is called titrant.

ii) Titrand

The volume of soln which is taken in a conical flask of unknown concentration is called titrand.

iii) End point

The point at which rxn complete is indicated by change in colour of indicator is called end point. It is a practical point.

iv) Equivalence point

The point at which equal amount of titrant is neutralized by titrand is called equivalence point. It is the theoretical end point.

The difference betn equivalence point or end point is called titration error.

v) Indicator

The chemical substance which indicate the completion of rxn by change in its own colour is called indicator. eg: methyl orange, methyl red, phenolphthalein.

Selection of indicator in acid-base titration

Principle of volumetric analysis

1) Equal volm of an acid is neutralized equal volume of base if their concn are same.

eg: 10ml of 0.1N HCl is neutralized 10ml of 0.1N NaOH.

100ml of 1N H₂SO₄ is neutralized 100ml of 1N KOH.

2) Normality and volm are reciprocal for the same soln.

eg: 1000ml of 1N Na₂CO₃ = 500ml 2N Na₂CO₃.

1000 × 5ml of 1N Na₂CO₃ = 5000ml of 0.2N Na₂CO₃.

1000/20 ml of 20N Na₂CO₃ = 50ml of 20N Na₂CO₃.

3) If an acid is neutralize by a base then N₁V₁ = N₂V₂.

Law of Equivalence (Normality equation)

let us consider an acid react with base.

At equivalence point:

No. of eq wt of an acid = No. of g eq wt of a base   …(1)

we know:

Normality × volume of soln in litre = no of eq wt of substance.

For acid:

For base:

Putting value of eqn (2) and (3) in eqn (1):

Normality of mixture

a) For similarly soln of mixture

b) For different soln of mixture

If N_mix +ve then the resulting soln is acidic.

If N_mix −ve then the resulting soln is basic.

If N_mix = 0 then the resulting soln is neutral.