Weak Acid Chemistry Handout

Weak Acid Chemistry Handout - pH. Weak acids. A....

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pH. Weak acids. A. Introduction. ................................................................................................... 1 B. Weak acids: overview. ................................................................................... 1 C. Weak acids: an example; finding K a .............................................................. 2 D. Given K a , calculate pH . ................................................................................. 3 E. A variety of weak acids. ................................................................................. 5 F. So where do strong acids fit in this picture? . ................................................. 6 G. More problems and questions. ....................................................................... 6 H. Polyprotic acids [briefly noted] . .................................................................... 7 I. Sulfuric acid [briefly noted] . ........................................................................... 8 J. Weak bases [briefly noted] . ............................................................................ 8 K. Answers . ........................................................................................................ 9 A. Introduction This handout follows Strong Acids. It leads to the final handout, on Buffers. In these last two handouts, there will be considerable emphasis on qualitative ideas rather than just calculations. B. Weak acids: overview Acids give off hydrogen ions. A good question is: what is the [H + ] (or pH) of an acid solution? Well, the first consideration must be: what is the acid concentration? 1 M HCl has more H + ions than 0.1 M HCl. But the next consideration is: how many H + ions do we get for each acid molecule? For strong acids, that is simple. By definition, strong acids ionize completely. Thus we can see how many H + we get per acid molecule just by looking at the chemical formula for the acid. (E.g., HCl gives one H + per HCl.) This was the subject of the preceding handout, on Strong Acids. Weak acids are not strong, are not completely ionized. Thus we need to specify how much ionization occurs (the “degree of ionization”). That is, we need to supply one more piece of information for weak acids. This is the basis of the additional complexity of dealing with weak acids. The amount of ionization of a weak acid is usually given by the equilibrium constant, K a . a relates to the ionization of a weak acid. They are the core of the new material here. Acid-base buffers, which are common, make use of the behavior of weak acids. Thus the discussion here about weak acids is a prelude to the discussion of buffers in the next handout.
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pH. Weak acids. Page 2 C. Weak acids: an example; finding K a Consider a 0.10 M aqueous solution of acetic acid. The following balanced equation describes how acetic acid ionizes in water: (1) CH 3 COOH (aq) H + (aq) + CH 3 COO - (aq) Acetic acid is a weak acid; it is only partially ionized. (The double arrow in Eq 1 shows that
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Weak Acid Chemistry Handout - pH. Weak acids. A....

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