Law of equivalence, Normality equation, Titration .pdf

Understand the basics of titration required for lab experiments.

1. 1

2. Law of equivalence & Normality equation
One gram equivalent of one chemical substance reacts exactly with one gram
equivalent of another substance.

ie For complete reaction, number of gram equivalents of reacting substances are
always equal.

This is called law of chemical equivalence.

eg
aA + bB → cC + dD

According to law of chemical equivalence,

no. of gm eq of A = no. of gm eq of B = no. of gm eq of C = no. of gm eq of D
(for complete reaction)

Chemical substances always react in equivalent proportions.

3. aA
(aq)
+ bB
(aq)
→ Products
For complete reaction
no. of gm eq of A (ng
1
) = no. of gm eq of B (ng
2
)
We have,
Normality = no. of gm eq / vol of solution in L
or, no. of gm eq = Normality x vol of solution in L
So, for A, ng
1
= N
1
V
1,

and for B, ng
2
= N
2
V
2

Therefore,
N
1
V
1
= N
2
V
2

This is called Normality equation.
Molarity equation
b(M
1
V
1
) = a(M
2
V
2
)?
If a = b
M
1
V
1
= M
2
V
2

4. 4

5. Titration
Titration is the method of quantitative chemical analysis which involves the
determination of volume of standard solution to react with measured volume of
solution of unknown concentration.

Also called Volumetric analysis
or Titrimetric analysis

Standard solution
The solution of known concentration is called standard solution.

6. Titration apparatus

7. Titration setup

8. Titrant
The solution taken in a burette is called titrant.

Titrand
The solution taken in a conical flask is called titrand.

Titre value
The solution consumed from burette. ie burette reading.

Equivalence point
The point in titration at which equivalent quantity of titrand is exactly neutralized by
titrant is called equivalence point or theoretical end point or stoichiometric end point.
However this point is not visible in acid base titration.
So indicator is use to indicate the completion of reaction between titrant and titrand.

Indicator
The auxiliary substance which is used to indicate the completion of reaction by the
sharp change in colour of solution or of precipitate is called indicator.

9. The point at which indicator shows the completion of reaction between titrant and
titrand by the sharp visual change in colour or the appearance of turbidity is
called end point of titration.

In the ideal titration, end point coincides with the equivalence point
but in practice the value of end point is slightly different from the value of
equivalence point.

Titration error
The difference between end point and equivalence point of titration is called
titration error.

In titration, indicator should be chosen in such a manner that it would avoid or
minimize the titration error.
End point

11. Types of titration
1.Acid-base titration or neutralization titration (acidimetry & alkalimetry)

HCl + NaOH → NaCl + H
2
O

Na
2
CO
3
+ 2HCl → 2NaCl + H
2
O + CO
2


Indicator used- Acid base indicator or pH indicator
eg phenolphthalein, methyl orange etc
(detail study- later)

2.Redox titration

Permanganometric titration

KMnO
4
+ H
2
SO
4
+ H
2
C
2
O
4
→ K
2
SO
4
+ MnSO
4
+ H
2
O + CO
2


KMnO
4
as self-indicator

Another eg-
Dichrometry titration
Iodometric titration (iodometry)
Iodimetric titration (iodimetry) etc

12. 4.Complexometric titration
Ca
++
+ disodium EDTA → Calcium EDTA complex + Na
+


Indicator used- Eriochrome black-T

5.Precipitation titration

NaCl + AgNO
3
→ AgCl ↓ + NaNO
3


Indicator used- potassium chromate solution

At end point:

2Ag
+
+ CrO
3
2-
→ Ag
2
CrO
4
(red)

13. Selection of pH indicator (acid-base indicator)

15. Colour of indicators in different media
Indicators Colour in
Acidic medium Basic medium
Litmus paper
Red 6 -7 Blue
Methyl orange
Pink or Red 3.2 - 4.4 Light yellow
Phenolphthalein
Colourless 8 - 10 Pink

21. 21

24. Past Questions
1.Define the terms (a) Titration (b) Volumetric analysis (c) Acidimetry (d) Alkalimetry (e) Titrant (f)
Titrand (g) Titre value (h) Equivalence point of reaction (i) Indicator (j) Endpoint of titration (k)
Titration error (l) Primary standard solution (l) Secondary standard solution (m) Normality factor
(n) unknown solution (o) semi normal solution (p) Equivalent weight of an element (q) Acidity of
base (r) Basicity of acid (s) redox titration (t) Molarity (u) Normality (v) Gram equivalent weight
(w) Neutralization point (x) Gravimetric analysis (y) Molar solution
2.Distinguish between decinormal and decimolar solution.
3.What is normality? How is it related to molarity?
4.It is better to express concentration of solution in molality rather than molarity, why?
5.Why is crystal of oxalic acid regarded as a good substance for the preparation of primary
standard solution?
6.What is the importance of of calculating normality factor of solutions during titration?
7.Describe different types of titration.
8.Write an example of redox titration. Why is it called so?
9.Indicator is not used in permanganometric titration. Why?
10.What is pH range of indicator? Write the values of pH range of different indicators.
11.How is end point differed from equivalence point of reaction?
12.What is indicator? Explain how are indicators selected in acid base titration?(For 5 marks)
13.In a titration of Na
2
CO
3
and HCl, which indicator can be used? Explain. (2 marks)
14.Phenolphthalein can not be used in the titration between ammonium hydroxide and HCl. Why?
(2 marks)