Ch_322a_3.07

Ch_322a_3.07 - Acid and Base Strengths: Ka and pKa Organic...

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Acid and Base Strengths: K a and pK a Organic acids (carboxylic acids) typically are weaker acids than the mineral acids (HCl, H 2 SO 4 ). While the latter dissociate completely in water, carboxylic acids, such as acetic acid, dissociate only to a small degree . CH 3 CO-H O = + H 2 O CH 3 CO - O + H 3 O + In a 0.1 M solution of acetic acid in water at 25 o C, only about 10% of the acetic acid molecules are dissociated to the acetate and hydronium ions.
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Acidity Constant, K a An equilibrium is established for the dissociation of acetic acid in water with an equilibrium constant, K eq , that is expressed as K eq = [H 3 O + ] [CH 3 CO 2 - ] [CH 3 COOH] [H 2 O] However, for dilute solutions, the concentration of water (~55.5 M ) does not change significantly during the reaction (the activity of water a = 1) , and a new equilibrium constant is defined: the acidity constant, K a . K a = K eq [H 2 O] = [H 3 O + ] [CH 3 CO 2 - ] [CH 3 COOH] Values of K a are tabulated. For acetic acid, at 25 o C, K a = 1.76 x 10 -5 .
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Ch_322a_3.07 - Acid and Base Strengths: Ka and pKa Organic...

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