Ka vs. Kb

The Conjugate Seesaw

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Christine Pahel 1C
Posts: 10
Joined: Fri Sep 25, 2015 3:00 am

Ka vs. Kb

Postby Christine Pahel 1C » Wed Dec 02, 2015 10:40 pm

How do you determine when to use Ka or Kb, especially when both a base/acid and its conjugate are present in the equation?

Nathan Danielsen 1G
Posts: 17
Joined: Fri Sep 25, 2015 3:00 am

Re: Ka vs. Kb

Postby Nathan Danielsen 1G » Wed Dec 02, 2015 11:10 pm

Whether you use Ka or Kb depends on what the problem is asking you to do. If in the equation the reactants have an acid mixing with water then you use Ka. If it is a base mixing with water, you use the Kb. If you are not given an equation, you must use your best judgement to determine whether the compound in question is being treated as an acid or a base in the problem. If it is being treated like an acid, use Ka, and if it is being treated as a base, use Kb. Also remember that if you end up flipping the equation, you must use the other K, as the conjugate is now the reactant. For instance, if you flip an equation that has an acid in the reactants, that reaction will now have the acid's conjugate base as a reactant, and you must switch from using Ka to Kb.

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