Dorothy M. Goldish
California State College o t Long Beach long Beach, California 90801
component concentrations in Solutions of Weak Acids
Textbooks usually present the subject of weak acids in terms of an algebraic equation. Using the equation, students make calculations, usually of the pH of a solution containing the specified acid. Evcn those students who acquire some facility a t making this calculation are left with no real feeling for the meaning of their answers. Some textbooks for non-science majors, recognizing the difficulty,omit the whole topic. I n practice, the calculation of the pH is often less useful than the estimation of the relative amounts of the various species present. I n particular, a study of reaction mechanisms or of biological systems requires knowledge of whether a given substance is or is not protonated in a given mixture. Few students learn to make snch an estimation as undergraduates. (One reason might be that everyone assumes they have learned it in another course!) The result is the sorry spectacle of graduate students undertaking research with no idea of the nature of the substances they are dealing with. A new approach to teaching the subject of weak acid equilibria has been used successfully with students of nursing and of home economics a t California State College a t Long Beach. Since these students often have difficulty with mathematics, the emphasis is on Le Chatelier's Principle, rather than on the mathematical expression of the equilibrium constant. The same approach, used in conjunction with the usual algebra, has proved useful for chemistry majors. The following explanation is used. A weak acid, symbolized HA, exists in solution as the equilibrium mixture1 HA + Ha0 s HsO+ A-
+
If the concentration of H30+ should be increased by adding another source of hydronium ion, the equilibrium wonld shift to the left to use up the hydronium ion. The result would be that some of the A- wonld also be used up, forming more HA. If bhe concentration of hydronium ion should be decreased (as by adding a base to react with it) the equilibrium would shift to the right,
' I t is often convenient to write H + rather than HaO+to focus attention on the proton, especially since acid-base reactions can certainly occor in the absence of water. The use here of HIO+ emphasizes the conjugate acid-base argument. For chemistry majors who are likely to fall into the habit sooner or later of writing H+, there is some advantage to be gained from bhe emphasis on the fact that the usual K. refers specifically to the transfer of a proton to water. The numerical value of K. is different in other solvents. The acid constant for the conjugate acid is given by K,/Ks, where K , = lo-'' a t 25% The use of I
