pH to PKa

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Mrudula Akkinepally
Posts: 105
Joined: Wed Sep 30, 2020 9:31 pm

pH to PKa

Postby Mrudula Akkinepally » Thu Jan 14, 2021 8:47 pm

Hello everyone!

During Justin's step-up section this morning, he said that we need additional information to be able to solve for pKa given the pH and pH given pKa.

I was wondering what that additional information is.

Please let me know!

Thank you :)

Mikayla James 2A
Posts: 90
Joined: Wed Sep 30, 2020 9:31 pm
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Re: pH to PKa

Postby Mikayla James 2A » Thu Jan 14, 2021 9:00 pm

Hi! A balanced chemical equation would be a good piece of additional information to be able to solve for pKa given the pH. An example of this is question 6D.3 in the textbook: "When the pH of 0.10 M HClO2 was measured, it was found to be 1.2. What are the values of Ka and pKa of chlorous acid?" From here, you can create a balanced chemical equation to understand the ratio of HClO2 to H3O+ and CLO2- to create an ICE table, then calculate Ka = ([H3O+][ClO2])/[HClO2] and pKa. Hope this helps!

Tessa House 3A
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Re: pH to PKa

Postby Tessa House 3A » Thu Jan 14, 2021 9:06 pm

I am not sure about this but based on how I believe you would calculate pH from pKa or pKa from pH, you might need the concentration of some of the reactants or products at equilibrium or an initial concentration so that you can solve for equilibrium concentration. If you were given pKa, you could find Ka but wouldn't know [H3O+] unless you had some other concentrations to plug into the equilibrium equation. If you had the pKa and the concentration of other reactants and products other than H3O+ at equilibrium, you could calculate this concentration and therefore pH. If you had the initial concentration of reactants, you could as well. I think this is what you might have to be given. I hope this helps!

Mikayla Kwok 3K
Posts: 99
Joined: Wed Sep 30, 2020 9:51 pm

Re: pH to PKa

Postby Mikayla Kwok 3K » Fri Jan 15, 2021 4:30 pm

I think we will learn this later, but you can get pKa from pH and pH from pKa by rearranging the Henderson-Hasselbalch equation, pH = pKa + log([base]/[acid]).

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