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General Chemistry Principles, Patterns, and Applications, 2011

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causes electrons to be drawn from oxygen to the central atom, weakening the O–H bond <strong>and</strong> increasing<br />

the strength of the oxoacid.<br />

Careful inspection of the data in Table 16.4 "Values of p" shows two apparent anomalies: carbonic acid<br />

<strong>and</strong> phosphorous acid. If carbonic acid (H2CO3) were a discrete molecule with the structure (HO)2C=O, it<br />

would have a single terminal oxygen atom <strong>and</strong> should be comparable in acid strength to phosphoric acid<br />

(H3PO4), for which pKa1 = 2.16. Instead, the tabulated value of pKa1 for carbonic acid is 6.35, making it<br />

about 10,000 times weaker than expected. As we shall see in Section 16.6 "Buffers", however, H2CO3 is<br />

only a minor component of the aqueous solutions of CO2that are referred to as carbonic acid. Similarly, if<br />

phosphorous acid (H3PO3) actually had the structure (HO)3P, it would have no terminal oxygen atoms<br />

attached to phosphorous. It would therefore be expected to be about as strong an acid as HOCl (pKa=<br />

7.40). In fact, the pKa1 for phosphorous acid is 1.30, <strong>and</strong> the structure of phosphorous acid is<br />

(HO)2P(=O)H with one H atom directly bonded to P <strong>and</strong> one P=O bond. Thus the pKa1 for phosphorous<br />

acid is similar to that of other oxoacids with one terminal oxygen atom, such as H3PO4. Fortunately,<br />

phosphorous acid is the only common oxoacid in which a hydrogen atom is bonded to the central atom<br />

rather than oxygen.<br />

Table 16.4 Values of pKa for Selected Polyprotic Acids <strong>and</strong> Bases<br />

Polyprotic Acids Formula p K a1 p K a2 p K a3<br />

carbonic acid* “H2CO3” 6.35 10.33<br />

citric acid HO2CCH2C(OH)(CO2H)CH2CO2H 3.13 4.76 6.40<br />

malonic acid HO2CCH2CO2H 2.85 5.70<br />

oxalic acid HO2CCO2H 1.25 3.81<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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