Which of the following acids have the largest pKa?
Correct Answer: A.CH3CO2H
Recall that pKa = -log Ka so their relative values, because of the negative sign in the equation, are always opposite to each other. Thus in an acid base equilibrium, AH <--> A- + H+, the largest pKa value is associated with the weakest acid. The unhalogenated carboxylic acid A is the weakest acid (acetic acid is the IUPAC name; ethanoic acid is the systematic way to name it). In the other cases, the electron withdrawing halogen substituent stabilize the carboxylate conjugate base (anion) making the parent acid more acidic.
The Medical College Admissions Test (MCAT) is a test administered by the Association of American Medical Colleges (AAMC). The test is designed to test your knowledge in three specific sections; physical sciences such as physics and general chemistry, biological sciences and organic chemistry, and verbal reasoning.
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