Normality
Normality is another way of expressing the concentration of a solution. It
is based on an alternate chemical unit of mass called the equivalent
weight. The normality of a solution is the concentration expressed as the
number of equivalent weights (equivalents) of solute per liter of solution.
A 1 normal (1 N) solution contains 1 equivalent weight of solute per liter
of solution. Normality is widely used in analytical chemistry because it
simplifies many of the calculations involving solution concentration.
Every substance may be assigned an equivalent weight. The equivalent
weight may be equal to the formula weight (molecular weight, mole
weight) of the substance or equal to an integral fraction of the formula
weight (i.e., molecular weight divided by 2, 3, 4, and so on). To gain an
understanding of the meaning of equivalent weight, consider the
following two reactions:
HCl(aq) + NaOH(aq) NaCl(aq) + H2O
1 mole 1 mole
(36.5 grams) (40.0 grams)
H2SO4(aq) + 2 NaOH(aq) Na2SO4(aq) + 2 H2O
1 mole 2 moles
(98.1 grams) (80.0 grams)
1 mole of hydrochloric acid (HCl) reacts with 1 mole of sodium hydroxide
(NaOH) and 1 mole of sulfuric acid (H2SO4) reacts with 2 moles of NaOH.
If you made 1 molar solutions of these substances, 1 liter of 1 M HCl will
react with 1 liter of 1 M NaOH and 1 liter of 1 M H2SO4 will react with 2
liters of 1 M NaOH. Therefore, H2SO4 has twice the chemical capacity of
HCl when reacting with NaOH. One can, however, adjust these acid
solutions to be equal in reactivity by dissolving only 0.5 moles of H2SO4
per liter of solution. By doing this, one is required to use 49.0 grams of
H2SO4 per liter (instead of 98.1 grams per liter) to make a solution that is
equivalent to one made from 36.5 grams of HCl per liter. These weights,
49.0 grams of H2SO4 and 36.5 grams of HCl, are chemically equivalent
and are known as equivalent weights of these substances because each
will react with the same amount of NaOH (40.0 grams). The equivalent
weight of HCl is equal to its molecular weight, but that of H2SO4 is ½ its
molecular weight. The table below summarizes these relationships:
Volumes Volumes
Formula that Equivalent that
weight concentration react weight concentration react
HCl 36.5 1 M 1 L 36.5 1 N 1 L
NaOH 40.0 1 M 1 L 40.0 1 N 1 L
H2SO4 98.1 1 M 1 L 49.0 1 N 1 L
NaOH 40.0 1 M 2 L 40.0 1 N 1 L
Normality 2
Expressions for normality are shown below. Notice the similarity to molar
solution definition.
normality N
number of equivalents of solute equivalents
=
1 liter of solution liter
where
number of equivalents of solute
grams of solute
=
equivalent weight of solute
then
N
grams of solute grams
= =
eq wt solute × L solution eq wt × L
So, 1 liter of solution containing 36.5 grams of HCl would be 1 N, and 1
liter of solution containing 49.0 grams of H2SO4 would also be 1 N. A
solution containing 98.1 grams of H2SO4 (1 mole) per liter would be 2 N
when reacting with NaOH in the above equation.
Consider the following reactions in which an excess of HCl is present.
Hydrogen actually exits as H2 molecules, but for convenience in
considering the data, the hydrogen produced is shown as the number of
atomic weights of hydrogen released per atomic weight of metal reacting.
Na(s) + HCl(aq) NaCl(aq) + H°(g)
Ca(s) + 2 HCl(aq) CaCl2(aq) + 2 H°(g)
Al(s) + 3 HCl(aq) AlCl3(aq) + 3 H°(g)
The table below summarizes the pertinent data for these reactions:
atomic number of atomic weights
weight of hydrogen liberated per equivalent weight
metal (amu) atomic weight of metal of metal (amu)
Na 23.0 1 23.0/1 = 23.0
Ca 40.1 2 40.1/2 = 20.0
Al 27.0 3 27.0/3 = 9.0
In each of the reactions, the equivalent weight of the reacting metals is
the weight that reacts with 1 equivalent weight of the acid, liberates 1
atomic weight of H atoms, or involves the transfer of 1 mole of electrons
in the reaction. One atomic weight of Na metal lost 1 electron per atom
going to NaCl; 1 atomic weight of Ca metal lost 2 electrons in going to
CaCl2; 1 atomic weight of Al metal lost 3 electrons in going to AlCl3. In
each reaction, 1 atomic weight of
+ H
gained 1 electron per atom in going to free hydrogen.
Normality 3
eq wt
at wt Na at wt Ca at wt Al at wt H
= = = =
1 2 3 1
Two definitions of equivalent weight can now be stated:
1. The equivalent weight is the weight of a substance that will react
with, combine with, contain, replace, or in any other way be
equivalent to 1 gram-atomic weight of hydrogen.
2. In oxidation-reduction reactions the gram-equivalent weight is the
weight of a substance that loses or gains 1 mole of electrons.
The equivalent weight of a substance may be variable; its value is
dependent on the reaction that the substance is undergoing. Consider
the following reactions:
NaOH + H2SO4 NaHSO4 + H2O
2 NaOH + H2SO4 Na2SO4 + 2 H2O
In the first reaction, 1 mole of sulfuric acid furnishes 1 gram-atomic
weight of hydrogen. Therefore the equivalent weight of sulfuric acid is the
formula weight (98.1 grams). In the second reaction, the equivalent
weight of sulfuric acid is ½ the formula weight (49.0 grams

Thanks & Regards
Rinkoo Gupta
AskIITians faculty