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ERIC Number: EJ1145979
Record Type: Journal
Publication Date: 2017-Jun
Pages: 6
Abstractor: As Provided
ISSN: ISSN-0021-9584
pK[subscript a] Values in the Undergraduate Curriculum: What Is the Real pK[subscript a] of Water?
Silverstein, Todd P.; Heller, Stephen T.
Journal of Chemical Education, v94 n6 p690-695 Jun 2017
Since at least the 1960s, organic chemistry textbooks have featured pK[subscript a] tables for organic acids that include values for H[subscript 2]O and H[subscript 3]O[superscript +] (15.74 and -1.74, respectively) that are thermodynamically and chemically indefensible. Here we trace this error back to Brønsted's early contributions in the 1920s to the Brønsted-Lowry Theory of acids and bases. Organic chemists generally defend the use of these values by citing measurements of the equilibrium constant for the water + methoxide acid-base reaction that suggested that methanol (pK[subscript a] = 15.54) is a stronger acid than water; from this, organic chemists have concluded that pK[subscript a] of water must be 15.74 rather than 14.00. Here we discuss the problems that invalidate this conclusion, the most important being that it is based on the use of the pure liquid standard state (mole fraction = 1) that is quite different from the standard state for acidities determined in dilute solution (molality = 1). Using the latter standard state, the equilibrium constant for the water/methoxide reaction ranges from 4 to 70, showing water to be a "stronger" acid than methanol, and justifying the use of the thermodynamically correct value, pK[subscript a](H[subscript 2]O) = 14.00.
Division of Chemical Education, Inc and ACS Publications Division of the American Chemical Society. 1155 Sixteenth Street NW, Washington, DC 20036. Tel: 800-227-5558; Tel: 202-872-4600; e-mail:; Web site:
Publication Type: Journal Articles; Reports - Evaluative
Education Level: Higher Education; Postsecondary Education
Audience: N/A
Language: English
Sponsor: N/A
Authoring Institution: N/A
Grant or Contract Numbers: N/A