pKa and pKb

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Michelle Song 1G
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Joined: Wed Nov 14, 2018 12:23 am

pKa and pKb

Postby Michelle Song 1G » Mon Jan 21, 2019 12:43 am

The textbook says that the weaker the acid, the smaller the value of Ka and the greater the value of pKa. If I'm interpreting this correctly, it makes sense that a weak acid would have a greater number on the pH scale because the lower the number the more acidic and the greater the number the more basic. However, the textbook also says that the weaker the base, the smaller the value of Kb and the greater the value of pKb. I'm confused because I thought that a weak base would have a lower value on the pH scale. How do pKa/pKb relate to pH?

Timothy_Yueh_4L
Posts: 57
Joined: Fri Sep 28, 2018 12:28 am

Re: pKa and pKb

Postby Timothy_Yueh_4L » Mon Jan 21, 2019 12:54 am

The pKa/pKb have no direct relation to the actual pH of a solution, however you use the values pKa and pKb to determine the hydronium and hydroxide ions of their respective reaction and use those values to determine the pH.

Aarti K Jain 1L
Posts: 30
Joined: Fri Sep 28, 2018 12:20 am

Re: pKa and pKb

Postby Aarti K Jain 1L » Mon Jan 21, 2019 10:03 am

With regards to Kb, one is looking at the pOH scale instead of the pH scale because OH- ions are produced, not H3O+ during the dissociation/reaction of a base. Therefore, on the pOH scale, having a greater value means that the base is weak.

KimGiang2F
Posts: 31
Joined: Fri Sep 28, 2018 12:19 am

Re: pKa and pKb

Postby KimGiang2F » Tue Mar 12, 2019 4:09 am

The pKa gives you the acidity value of a substance and the Ka value indicates that the amount of acid that actually dissociates. While the pkb gives you the basicity value of a substance and the Kb value determines the level of dissociation of a base. For example, if given the pKa, you can use 10^-pKA to get the Ka. Essentially to solve for the [H+] or x value, you set the Ka equal to [products]/[reactants]. To find the pH, you then take the -log of [H+]. The pKa gives you the acidity value of a substance and the Ka value indicates that the amount of acid that actually dissociates. The same is for pkb and Kb, except they're used to find the pOH.


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