Ka and Kb/pOH and pH use in calculations

[Gabi_Maldonado_1B]

I noticed myself getting tripped up over the Ka and Kb values and using the wrong ones on this weeks Achieve HW. Can someone please clarify if you should be using strictly Kb, pOH and [OH-] values in order to compute values for bases and Ka, pH and [H3O+] for acids (excluding the use of pOH to find the pH and Ka to find Kb)? (I am referencing calculations using the ice box.)

[Emma Kelly 3L]

I believe it depends on what information you're given in the problem rather than whether you're dealing with acids or bases; for example, if you're given Kb values, you would find pKb and pOH even if you're talking about acids. You can then use these values to find Ka, pKa, and pH if you need to.

Hope this helps!

[Brandon Yu]

You are correct. Sometimes however, questions will try to trick you by giving say a Kb value but your dealing with an acid. This will force you to convert the given Kb to Ka so that you can easily calculate pH, [H+], etc. as you stated!

[Travis Wang 2G]

Kb values go hand in hand with base reactions and Ka values go hand in hand with acid reactions. Like Brandon said, you might be given a Kb value for an acid reaction, but writing out the chemical reaction and seeing how the reactants form products helps to determine which value you need to use. I'm not sure if you can definitively eliminate one set of values for acids and the other for bases since they're all so intertwined.

[Bela Patel 2B]

As said, it fully depends on what you are given in the question. If you are talking about bases you use, Kb, pKb, and pOH, but if you talking about acids you use, Ka, pKa, and pH. However, if you are given acid units and are asked to find the base then you would need to convert the values.

[Arden Napoli 1E]

Hi,

You have correctly associated the right variables with acids and bases, however it is dependent on what the question gives you. For some of the problems on the Achieve, you will notice that they may ask you for the pOH, however, you are starting with the pKa. Therefore, it is important to pay attention to the details of the question so you can plan if you need to convert.

[Anna Furton]

I agree with the replies above, and I would say make sure to keep an eye on whether the information you're given in the question signifies an acid to conjugate base reaction, or a base to conjugate acid reaction. To make an ICE table for an acid to conjugate base reaction, you will want to make sure you're using Ka, pKa, pH etc, and the opposite goes for base to conjugate acid ICE tables.

It isn't difficult to convert between the different terms:
Ka x Kb = Kw = 10^-14
pKa + pKb = 14
pH + pOH = 14

[Albert Chen 1G]

Hello, I also believe it is dependent on what you are given and what the question is asking. To start with a question, you can be given Kb, but are asked for pH. It is up to you whether you want to convert Kb to Ka in the beginning or simply do calculate using Kb and convert to pH later. So, you are given freedom in which way you want to solve it.

[Evonne Chau 2I]

Hi Gabi,

You're correct. Ka, pH, and [H+] values pertain to acids while Kb, pOH, and [OH-] pertain to bases. These values could be use to find each other.
Kw = Ka x Kb where Kw = 1.0 x 10^-14
Ka = [P]/[R] when dealing with acids
Kb = [P]/[R] when dealing with bases
pH + pOH = 14
pH = -log[H+]
pOH = -log[OH-]

Sometimes, the questions might try to throw you off by giving you the Ka value for a base reaction or the Kb value for an acid reaction. Because of this, you're going to have to convert them.