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Thermo Kin Lecture 9

Thermo Kin Lecture 9 - Lecture 9 Acid-Base Equilibria...

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Acid-Base Equilibria HCl (aq) + H 2 O (l) H 3 O + (aq) + Cl - (aq) or HCl (aq) H + (aq) + Cl - (aq) But we also have this reaction in water: H 2 O (l) OH - (aq) + H + (aq) K w = [OH - (aq) ] [H + (aq) ] = 1.0x10 -14 M 2 at 25ºC, ideal solution Acid (donates proton) Conjugate base (can accept proton) () () () ] [HCl ] ][Cl [H K aq aq aq a + = For a strong acid, K a is very large, i.e. it is completely dissociated Lecture 9

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Example: strong acid What is the concentration of ions in a solution of 0.01 mol/L (or 0.01 M) HCl? Strong acid fully dissociates: 0.01 M HCl -> 0.01 M H + + 0.01 M Cl - In water at 25ºC, we must have [OH - ] [H + ] = K w =1.0x10 -14 M 2 (ideal soln) Initially (before acid is added) we have [OH - ] = [H + ] = 1.0x10 -7 M Finally we have [H + ] = (0.01 – x ) + 10 -7 [OH - ] = (– x + 10 -7 ) [OH - ] [H + ] = (– x + 10 -7 ) ((0.01 – x ) + 10 -7 ) = 1.0x10 -14 M 2 Find x = 0.99999x10 -7 M, so [H + ] = 10 -2 M [OH - ] = 10 -12 M Define pH = -log 10 a H+ = -log 10 [H + ] for an ideal solution
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Thermo Kin Lecture 9 - Lecture 9 Acid-Base Equilibria...

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