Page 12 - PC 101 Interactive practical book

MANSOURA NATIONAL UNIVERSIY
PHARM D- CLINICAL PHARMACY LEVEL I PHARM. ANAL. CHEM. I (PC 101)

General definition of the equivalent weight of a substance:

It is the weight of the substance that is equivalent in its reactive power

to 1 mole of hydrogen (i.e. the weight of the substance that will combine with

or replace 1 mole of hydrogen).

The specific definition and calculation of equivalent weight differ

according to the type of the reaction. Now, it will be studied in acid-base

reactions while other reactions will be discussed later.

Definition of Equivalent Weight in acid-base reactions:

It is the weight of the substance that will release, react with or

+
be chemically equivalent to 1 mole of hydrogen ions (H ) in that reaction.

How to calculate it?

a) For acids: Equiv. wt. = M. W. of the acid
no. of replaceabl e H  in one molecule of the acid

ex: equiv. wt. of H2SO4 = M.W. of H2SO4/2 = 98/2 = 49 g H2SO4.

M. W. of the base
no. of replaceabl e OH in one molecule of the base
b) For bases: Equiv. wt. = 
ex: equiv. wt. of NaOH = M.W. of NaOH/1 = 40/1 = 40 g NaOH.

c) For salts: Equiv. wt. = M. W. of the salt
no. of H  equivalent to one molecule of the salt
ex: equiv. wt. of CaCO3 = M.W. of CaCO3 / 2

because CaCO3 + 2 HCl CaCl2 + H2O + CO2

+
(i.e. 1 CaCO3 is equivalent to 2 H ).

N.B. In case of salts the number by which we divide the M.W. of the salt is

usually the sum of +ve charges on the cation radical OR the sum of –ve

charges on the anion radical of the salt.

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