The Need for Normality
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Carl Schaumburg Von Steuben M.S.C.
5039 N. Kimball Ave.
Chicago, IL 60625
The materials here will show that not all salts are neutral and will
demonstrate the necessity for measuring concentration using normality in
Small amounts of solid salts which have a variety of pH's such as NaCl,
Na3PO4, K2SO4, KNO3, NH4NO3, NH4Cl and whatever others you have available,
pHydrion paper (1-12), solutions of phenolphthalein, bromthymol blue, alizarin,
solutions of 0.1M sodium hydroxide, 0.1M hydrochloric acid, 0.1M sulfuric acid,
0.1M phosphoric acid, burets, pipets and other glassware for titrations.
Start by reviewing reactions of acids and bases. When someone mentions
neutralization, STOP and discuss this reaction. Using small amounts of solid
salts and pHydrion paper discover and discuss that salts are not all neutral.
When acids react with bases, the final pH will vary.
Have students add a squirt of bromthymol blue, phenolphthalein and alizarin
to separate test tubes. Add 4 drops of 0.1M HCl to each. Neutralize the acid
by adding 3 or 4 drops of 0.1M NaOH to the bromthymol blue until it turns green
or just barely turns blue. Counting the drops of base, repeat the process with
phenolphthalein until a light pink color appears and with alizarin until a
purple appears. Discuss! Indicators change colors at various pH values. It
should take more base for the second two and this gives an indication of the
higher pH at which they change color. Which indicator will be best for salts
with pH of 7, 9 or 12?
Finally three titrations are done in the normal fashion. Everyone will use
10 mL of acid but some groups should use 0.1M hydrochloric acid, others 0.1M
sulfuric acid and the rest 0.1M phosphoric acid. All titrations will use 0.1M
NaOH. Since all acids were the same concentration, what are predictions of the
amount of base solution needed? How much base solution does it actually take?
Why? Investigation of the number of H's in the formula of the acid should give
the clue. Two H's will neutralize twice the volume of base and three H's three
times. Lead everyone to see that the concept of molarity is not sufficient for
many titrations. A new method of expressing concentration is required -
normality. Now the definition, discussion and usual problems sets can be done.
A quiz or discussion of the understanding of pH of salts could determine if
this concept is understood. Practice titration problems with polyprotic acids
would quickly show those who do not understand.