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The Base Dissociation Constant, K b

The Base Dissociation Constant, K b. The Base Dissociation Constant. A weak base, represented by B, reacts with water to form an equilibrium solution of ions. B (aq) + H 2 O ( l ) HB + (aq) + OH - (aq) The equilibrium expression for this general reaction is given as follows:

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The Base Dissociation Constant, K b

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  1. The Base Dissociation Constant, Kb

  2. The Base Dissociation Constant • A weak base, represented by B, reacts with water to form an equilibrium solution of ions. B(aq) + H2O(l) HB+(aq) + OH-(aq) • The equilibrium expression for this general reaction is given as follows: Kb = [HB+][OH-] [B]

  3. Some Base Dissociation Constants at 25oC

  4. Sample Problem • The characteristic taste of tonic water is due to the addition of quinine. Quinine is a naturally occurring compound that is also used to treat malaria. The base dissociation constant for quinine is 3.3 x 10-6. Calculate [OH-] and the pH of a 1.7 x 10-3 mol/L solution of quinine. • What is required? • You need to find [OH-] and pH • What is given? • Kb = 3.3 x 10-6 • [Quinine] = 1.7 x 10-3 mol/L

  5. Sample Problem • Step 1 Q(aq) + H2O(l)HQ+(aq) + OH-(aq) Initial 1.7 x 10-3 0 0 Change -x +x +x Equil (1.7 x 10-3) - xxx • Step 2 Kb = [HQ+][OH-] [Q] 3.3 x 10-6 = (x)(x) (1.7 x 10-3) - x

  6. Sample Problem • Step 3 [Q] = 1.7 x 10-3 = 515 Kb 3.3 x 10-6 Since the value is greater than 500, the amount that dissociates is probably negligible, compared with the initial concentration of the base. • Step 4 3.3 x 10-6 = x2 1.7 x 10-3 x = 7.5 x 10-5 mol/L = [OH-]

  7. Sample Problem • Step 5 pOH = -log[OH-] = -log(7.5 x 10-5) = 4.13 pH = 14 - pOH = 9.87

  8. Remember: Conjugate Acid-Base Pairs • Two molecules or ions that are related by the transfer of a proton are called a conjugate acid-base pair. • The conjugate base of an acid is the particle that remains when a proton is removed from the acid. • The conjugate acid of a base is the particle that results when the base receives the proton from the acid.

  9. Ka Values of Conjugate Acids of Bases • We have used Ka and Kb as the acidic and basic constants of acids and bases. • Can an acidic constant, Ka, be assigned to the conjugate acid of a base? • What is the relationship between Ka of the conjugate acid and Kb of the base?

  10. Ka Versus Kb • For any conjugate acid-base pair: Kax Kb = Kw

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