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Which of the following acids have the largest pKa?

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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.
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