# The actual ph of a solution of a weak acid can be

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The actual pH of a solution of a weak acid can be calculated from the known concentration of the weak acid. For example, if C o moles of a weak acid are dissolved in water and x moles of protons are released from the acid then the concentration of the various species is: [HA] = C o - x [H + ] = x [A - ] = x From the definition of the acidity constant: We can solve for x under any conditions using the quadratic equation. However, this equation can be simplified if the concentration of the released protons is small compared to the concentration of the weak acid ( i.e. C o >> x), giving: Polyprotic Acids Many compounds can release more than one proton in the pH range of 0-12. Examples are phosphate (pK a = 2.14, 7.2, 12.4), carbonate, and dicarboxylic acids. If the pK a s are separated by 2 or more pH units ( i.e. phosphoric acid) each ionization can be treated separately. Depending on the structure of the polyprotic acid the pK a for each ionization can vary widely. Consider the following:

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7 Acid pK 1 pK 2 Phosphoric 2.14 7.20 Oxalic 1.23 4.19 Succinic 4.21 5.63 Carbonic 6.37 10.20 Zwitterionic Compounds Zwitterionic compounds can bear both a negative and a positive charge. Amino acids are examples of zwitterions: a. Titration curves show two ionizations. b. pK a1 = 2.3. c. pK a2 = 9.8. d. At pH << pK a1 , the charge is +1. e. At pH = 1/2(pK a1 + pK a2 ), the amino acid has no net charge. f. At pH >> pK a2 , the charge is -1. 8.31.04
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