5 acid pk a type hcl 7 very strong oxalic 123 strong

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5 Acid pK a Type HCl -7 Very strong Oxalic 1.23 Strong Acetic 4.76 Weak Ammonium (NH 4 + ) 9.25 Very weak Methanol 16 Extremely weak Benzene 30 Not really an acid 2.5 Titration Curves See the pH titration animation at: Phosphoric Acid (H 3 PO 4 ) Titration Curve shows a typical polyprotic acid titration. You should be able to identify all three: 1) pK a 's; 2) buffer ranges; and 3) equivalence points. 2.6 Buffers A pH buffer is an acid which resists changes in the solution pH. Buffers play an important role in cellular processes because they maintain the pH at an optimal level for biological processes. All acids are good buffers at pH values near (within one pH unit of) their pK a . Strong acids (such as HCl) are poor buffers while weaker acids (such as acetic acid) are good buffers in the pH ranges found in biological environments. Conversely, weaker acids (such as imidazole, pK a = 6.04) are good buffers in this range The reason for this is that strong acids are completely dissociated in this pH range while weak acids are not. For example, consider a 10 mM solution (1L) of HCl or acetic acid at pH 4.7. At this pH all of the HCl is ionized and exists as H + and Cl - . In contrast, only about 50% of the acetic acid is ionized. If we added 1 mmole of a strong acid ( i.e. HCl) to each solution the pH changes would be as follows: In the case of the HCl, all of the added acid would remain as H + ; thus, the H + concentration would increase from 10 -5 M to 10 -3 M, or the pH would drop to 3. In the case of the acetic acid solution, most of the added acid would simply protonate the acetate ions, reducing their concentration from 5 mM to 4 mM. The resultant pH of the solution is:
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6 The buffering capacity of a weak acid decreases as the dissociation becomes more complete. For example, acetate is a poor buffer at pH 3 or at pH 7 (see Figure 2.8).
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