Equivalent Weight of
an Unknown Acid

To find the equivalent weight of an unknown acid.

By definition one equivalent (or equivalent weight) of a substance is the amount of that substance which supplies or consumes one mol of reactive species. In acid-base chemistry the reactive species is the hydrogen ion (H1+) while in oxidation-reduction chemistry the reactive species is the electron. For example, in the following two reactions the equivalent weight of H2SO4 would be 49 grams or 0.5 mol in the first reaction but 98 grams or 1 mol in the second. On the other hand, sodium hydroxide has the same equivalent weight in both reactions, one mol or 40 grams.
(1) H2SO4 + 2NaOH ------> Na2SO4 + 2H2O
(2) H2SO4 + NaOH ------> NaHSO4 + H2O

In the first reaction one mol of H2SO4 supplies 2 mols of H1+ to NaOH, therefore, one-half mol of H2SO4 or 49 grams is one equivalent. The conditions are different in the second reaction because sulfuric acid only "looses" one hydrogen so the equivalent weight of sulfuric acid is one mol or 98 grams. However, sodium hydroxide behaves the same in both reactions, that is, one mol of sodium hydroxide always "consumes" one mol of H1+, so its equivalent weight remains the same at one mol or 40 grams.

In order to determine the equivalent weight of a substance you must know something about the reaction but it does not have to be balanced. Equivalents can help in the analysis of a substance when the balanced reaction is not known or cannot be written for whatever reason; because one equivalent always reacts with one equivalent. (You should prove this to yourself by calculating how much sodium hydroxide is needed to react with 49 grams of sulfuric acid in each of the two reactions above. Do your calculations using traditional mol relationships and the one to one relationship for equivalents.)

In this lab you will be determining the equivalent weight of an unknown acid by using a standardized solution of NaOH. Each partner is to titrate a different unknown acid. Be very careful as part of your grade will be based on accuracy.

1. Obtain the unknown acid from your instructor. Write down the code number on the vial. Prepare the sample by grinding it to a fine powder with a mortar and pestle.
2. Dissolve small portions of your unknown acid in distilled water and titrate with the standardized NaOH to a phenolphthalein endpoint.
3. Repeat the titration two more times.

1. Calculate the equivalent weight of your unknown acid. Report this along with the unknown's code number.

2. Calculate the mass of your unknown acid that would be required to neutralize 10.0 grams of Al(OH)3 assuming that the aluminum hydroxide reacted as follows:

Al(OH)3 ------- > Al(OH)2+

3. A better procedure would be to dry the unknown acid prior to titration. Why? What effect would this have on your calculated equivalent weight?

4. Polyprotic acids can undergo neutralization to produce both normal and acid salts. Calculate the equivalent weight of phosphoric acid for the production of each of the three possible phosphate salts.

Teacher Notes

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The "unknown acid" samples are prepared by mixing various amounts of an acid, like nicotinic acid or potassium acid phthalate, with some inert material like sodium or potassium chloride. Students should be told how the samples are prepared so that the "unknown acid" is sufficiently ground to insure that it is homogeneous.

Questions? Comments??