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If a weak acid is dissociating in solution, then you use Ka, which is the K value for a reaction in which an acid gives off a proton and adopts its conjugate base form. Kb is the opposite in that it is the K value for a reaction in which a base accepts a proton and becomes its conjugate acid, producing OH- in the process. You can use Ka and Kb depending on the direction of the reaction as well. For example, if a problem gives you the Ka and then asks you to calculate the concentration of OH-, you can calculate the Ka using Kw and use an ICE table to calculate the equilibrium concentrations of OH-.
Ka is the disassociation constant for a weak acid and for Kb its for a weak base. You typically use them in ICE tables to find how much an acid or base disassociates. If you are dealing with H+ or H30+ use Ka and for OH- use Kb
205150314 wrote:Just trying to reassure things before the midterm. When exactly can you tell that you would either need Ka or Kb?
If you have OH- being formed, you need Kb, if you have H+ or H30+, Ka.
Use Kw to convert between the two.
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