Lecture 10--Polyprotic Acids and Salts

Lecture 10--Polyprotic Acids and Salts - Ch 302 Notes...

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Ch 302 Notes 10/19/2009 B. A. Rowland 1. Polyprotic Acids Remember that a polyprotic acid is one which is capable of furnishing more than one proton per molecule. They will always dissociate in a step-wise manner (which means we can treat them with RICE tables). Phosphoric Acid: We can write three different ionization reactions for phosphoric acid, each with its own Ka value: Reaction 1: K a1 = 7.5e-3 = Reaction 2: K a2 = 6.2e-8 = Reaction 3: K a3 = 4.8e-13 = For most polyprotic weak acids, K a1 > K a2 > K a3 > … > K aN . Remember that K w = K a K b for any conjugate acid base pair!
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Example: Compute the pH and the concentration of all major species in a 5.0 M solution of phosphoric acid. Key: In general, you will treat all polyprotic acids just as you would a monoprotic weak acid—the second, third, etc. dissociations don’t really matter in the end!
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Sulfuric Acid: Sulfuric acid is unique in that it is the only acid for which the first dissociation is strong, while the second is weak (use a
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Lecture 10--Polyprotic Acids and Salts - Ch 302 Notes...

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