Strength of acids based on Ka and pKa

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Letizia Ye 4F
Posts: 62
Joined: Fri Sep 28, 2018 12:28 am

Strength of acids based on Ka and pKa

Postby Letizia Ye 4F » Wed Jan 23, 2019 4:48 pm

Hi!
How can we tell how strong an acid is by looking only at their Ka and pKa value? Does this rule work the same for bases? Thanks.

Felicia1E
Posts: 31
Joined: Fri Sep 28, 2018 12:22 am

Re: Strength of acids based on Ka and pKa

Postby Felicia1E » Wed Jan 23, 2019 4:51 pm

The rule is this: The larger the Ka, the stronger the acid. The larger the Kb, the stronger the base. The smaller the pKa, the stronger the acid. The smaller the pKb, the stronger the base.

Gillian Murphy 2C
Posts: 30
Joined: Fri Sep 28, 2018 12:22 am

Re: Strength of acids based on Ka and pKa

Postby Gillian Murphy 2C » Wed Jan 23, 2019 5:43 pm

pKa and pKb work on the same scale as pH and pOH, respectively. A low pKa indicates a stronger weak acid, whereas a high pKa indicates a weaker acid. In the same way, a low pKb indicates a stronger weak base, whereas a high pKb indicates a weaker base. You can look at the Ka and Kb to compare, but I personally find the pKa and the pKb scales much easier to visualize, and it's really easy to find them from the Ka and Kb (just take the -log of them!)

Irene Zhou 1E
Posts: 73
Joined: Fri Sep 28, 2018 12:18 am

Re: Strength of acids based on Ka and pKa

Postby Irene Zhou 1E » Wed Jan 23, 2019 8:17 pm

I agree with all the previous responses. The smaller the pKa, the larger the Ka, which means the stronger the acid. The same is also true for bases. The smaller the pKb, the larger the Kb, which means the stronger the base.

Cade Okohira 4K
Posts: 47
Joined: Fri Sep 28, 2018 12:15 am

Re: Strength of acids based on Ka and pKa

Postby Cade Okohira 4K » Sat Jan 26, 2019 11:24 pm

The strength of acids and bases is determined by their Ka and Kb, respectively. So the higher the Ka, the stronger the acid, and the higher the Kb, the stronger the base. However, for pKa and pKb, they work the opposite way: the higher the pKa, the weaker the acid, and the higher the pKb, the weaker the base. pKa and pKb were just created to form numbers that are easier to visualize and comprehend. However, you would usually want to convert to Ka/Kb, in which you simply take the -log of pKa or pKb.


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