12.8 What is the difference between Molarity and Normality? (Chemistry)
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This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
12.8 What is the difference between Molarity and Normality? (Chemistry)
A Molar solution contains one gram molecular weight ( aka mole ) of the
reagent in one litre of solution, and is represented by " M ". In modern
usage, "molar" is intended to only mean " divided by amount of substance",
and is not supposed to be used to describe 1M solutions. There are already
exceptions to the rule ( molar conductivity, molar extinction coefficient ),
so I would only worry about correct usage in exams, as in the real world
most chemists use Molar to describe 1M solutions.
A Molal solution is one gram molecular weight of the reagent in 1 kilogram
of solvent, and is usually represented by "m". This concentration unit is
relatively uncommon in the real world, so it's worth checking that the "m"
is not a "M" typo.
A Normal solution contains one gram equivalent weight ( aka equivalent )
of the reagent in one litre of solution, and is represented by " N ".
The equivalent weight of a reagent may vary according to the reaction, but
if considering just acid and base moles and equivalents, then:-
1M H2SO4 + 2M NaOH -> 2H2O + Na2SO4
1N H2SO4 + 1N NaOH -> H20 + 0.5Na2SO4
1N HCl + 1N NaOH -> H2O + NaCl
So you can see that the equivalent weight of an acid is that which contains
1.0078 grams of replaceable hydrogen which, in the case of sulfuric acid,
would be half the mole weight, but, in the case of hydrochloric acid, would
be the mole weight.
The equivalent weight of a base is that which contains one replaceable
hydroxyl group ( ie 17.008g of ionisable hydroxyl ). Thus the equivalent
weight of sodium hydroxide ( NaOH ) and potassium hydroxide ( KOH ) would
be the mole weight, but for calcium hydroxide ( Ca(OH)2 ) it would be half
the mole weight.
The equivalent weight of an oxidising or reducing agent is that weight of
the reagent that reacts with or contains 1.008 grams of available hydrogen
or 8.000 grams of available oxygen. "Available" means being able to be
utilised in oxidation or reduction reactions. The equivalent weight of an
oxidising agent is determined by the change in oxidation number which the
reduced element experiences, eg the reduction of potassium permanganate in
dilute H2SO4 gives;-
K Mn O4 --> Mn S O4
(Oxidation Number) +1 +7 -8 +2 +6 -8
This results in a change of the manganese from +7 to +2, so the equivalent
weight is 1/5 of a mole. However, in neutral solution the change would only
be 3 because the product is MnO2, giving an equivalent weight of 1/3 of a
mole. If reacted in strongly alkaline solution the product is MnO4--, giving
an equivalent weight of one mole.
The equivalent weight of a reducing agent is determined by the change in
oxidation number that the oxidised element undergoes. For the conversion of
ferrous sulfate into ferric sulfate;-
2 (Fe SO4) --> Fe2 (SO4)3
(Oxidation Number) 2x(+2 -2 ) (+3)x2 (-2)x3
The change in oxidation number per atom of iron is 1, so the equivalent
weight of ferrous sulfate is 1 mole.
There are wide range of rules about the determination of the oxidation
number, but if you have been taught to use molarity, I would not bother too
much about normality, as it is mainly used these days by analytical
chemists - because it is convenient for many common titrations. Analysts
assume that 1 ml of 1N reagent will react with 1 ml of 1N reagent. However,
there has been a recent Journal of Chemical Education article that claims
using normality and equivalent weight does help students understand
chemistry, but those concepts are unlikely to become widespread again [11].
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