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Dissociation Constant or Ionization Constant for Weak Acids & Bases

Dissociation Constant or Ionization Constant for Weak Acids & Bases.

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Dissociation Constant or Ionization Constant for Weak Acids & Bases

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  1. Dissociation Constant or Ionization Constant for Weak Acids & Bases When a weak acid, HX, is placed into water some of it reacts with water to form H3O+ and X- ions. An equilibrium is established, in which the dissociation reaction HX + H2O  H3O+ +X- occurs at the same rate as the association reaction, H3O+ +X- HX + H2O.

  2. We describe this situation by writing the reaction equation as: HX(aq) + H2O(l) H3O+(aq)+ X-(aq) When the law of chemical equilibrium is applied, the mass action expression we get is: Keq = [H3O+] [X-] [HX] Since Keq expresses the tendency for the acid to dissociate, it is often called the “dissociation constant” or “ionization constant” for the weak acid. symbolically: Kdiss or Ka weak acids Kdiss or Kb  weak bases

  3. NOTE: • If Ka, Kb, Kdiss > 10: The substance is essentially 100% dissociated for our purposes. This will happen with strong acids & bases like HCl or NaOH. • If Ka, Kb, Kdiss < 10: Dissociation is less extensive & the acid or base is said to be weak. • A knowledge of Kdiss for a weak acid or base is useful to chemists since it allows the [H3O+] & pH of the solution to be calculated.

  4. Ka Problems – Type I Finding the % dissociation & Kdiss given concentration If the pH of a 0.100 M solution of CH3COOH is 2.86, find • % diss • Ka

  5. Type II: Finding concentrations, % dissociation when given Ka or Kdiss Given a 0.900 M solution of a weak acid, HX, whose Ka is 4.5 x 10-4, find the [H3O+], pH & % dissociation.

  6. Type II with a twist Find the [H3O+] & pH of a solution which was made up to be 0.100 M HOCN (cyanic acid) & 0.0500 M in NaOCN (sodium cyanate). Assume complete dissociation. Ka = 3.47 x 10-4

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