Ka and Kb Formula

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TiffanyBrownfield 2I
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Joined: Wed Sep 30, 2020 9:42 pm

Ka and Kb Formula

Postby TiffanyBrownfield 2I » Sun Jan 24, 2021 10:02 pm

So when you're given Ka it says the formula is [H+][A-]/[HA]. In the video I watched it said you can simply write it as just x^2/[molarity - x] since [H+] and [A-] are equal. Is this statement always true?

Charmaine Ng 2D
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Re: Ka and Kb Formula

Postby Charmaine Ng 2D » Sun Jan 24, 2021 10:10 pm

I'm not completely sure, so it'd be great if someone could fact check me!:
We've mostly been working with monoprotic acids, so this holds true for most of the problems we've done. However, note that it depends on the chemical equation's coefficients. If, for example, two moles of A- are gained from each mole of HA lost, then A- would have -2x, which would make [H+] and [A-] different. If it's monoprotic like the problems we've been doing so far, you don't have to worry about this!

Bhuvan Kommineni 3L
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Re: Ka and Kb Formula

Postby Bhuvan Kommineni 3L » Sun Jan 24, 2021 10:11 pm

This is true for monoprotic acids.

Jose Miguel Conste 3H
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Joined: Sat Oct 03, 2020 12:15 am

Re: Ka and Kb Formula

Postby Jose Miguel Conste 3H » Sun Jan 24, 2021 10:13 pm

I think this is honestly true, but I would just always write [H+][A-], so then it sticks that you're multiplying the conjugate base and prevent any problems

Kandyce Lance 3E
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Re: Ka and Kb Formula

Postby Kandyce Lance 3E » Sun Jan 24, 2021 11:00 pm

What key words should we look for in order to determine if we should be using Ka and Kb? This concept caused me a few issues with the homework

Sarah Salam 1J
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Re: Ka and Kb Formula

Postby Sarah Salam 1J » Sun Jan 24, 2021 11:11 pm

Kandyce Lance 3E wrote:What key words should we look for in order to determine if we should be using Ka and Kb? This concept caused me a few issues with the homework

In the homework, you'd need to use Ka when solving for a weak acid, and Kb when solving for weak bases. You solve for one by dividing 1x10^-14 by the other. The processes for using them are similar.

Kaiya_PT_1H
Posts: 90
Joined: Wed Sep 30, 2020 10:07 pm

Re: Ka and Kb Formula

Postby Kaiya_PT_1H » Sun Jan 24, 2021 11:17 pm

Yeah I think this is true for monoprotic acids, which is mostly what we work with in class!

Mariah Disc 2C
Posts: 86
Joined: Wed Sep 30, 2020 9:38 pm

Re: Ka and Kb Formula

Postby Mariah Disc 2C » Sun Jan 24, 2021 11:20 pm

This will be true with the problems that we are focusing on in class!

Adrienne Chan 1G
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Re: Ka and Kb Formula

Postby Adrienne Chan 1G » Sun Jan 24, 2021 11:25 pm

This seems true for monoprotic acids!

Dane_Beasley_1E
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Re: Ka and Kb Formula

Postby Dane_Beasley_1E » Sun Jan 24, 2021 11:32 pm

That is true for all monoprotic acids.

Geethika Janga 1L
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Re: Ka and Kb Formula

Postby Geethika Janga 1L » Sun Jan 24, 2021 11:35 pm

I'm pretty sure this holds true for most monoprotic acids.

joshtully
Posts: 92
Joined: Wed Sep 30, 2020 9:36 pm

Re: Ka and Kb Formula

Postby joshtully » Sun Jan 24, 2021 11:40 pm

This is true for monoprotic acids.

Sable Summerfield
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Joined: Tue Nov 17, 2020 12:18 am

Re: Ka and Kb Formula

Postby Sable Summerfield » Sun Jan 24, 2021 11:47 pm

How do we know when we need to convert Kb to Ka or vice versa?

Sam Wentzel 1F 14B
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Re: Ka and Kb Formula

Postby Sam Wentzel 1F 14B » Sun Jan 24, 2021 11:50 pm

This statement is ALWAYS true of weak monoprotic acids / bases but you can make assumptions about the x value on the bottom of the expression if the KA is 10^-3 or less, such that x on the bottom roughly = 0, but x on the top STILL = x.

Queena Chu 3E
Posts: 86
Joined: Wed Sep 30, 2020 10:09 pm

Re: Ka and Kb Formula

Postby Queena Chu 3E » Sun Jan 24, 2021 11:51 pm

Yep, I'm pretty sure its true for monoprotic acids.

Susan Chamling 1F
Posts: 91
Joined: Wed Sep 30, 2020 9:42 pm

Re: Ka and Kb Formula

Postby Susan Chamling 1F » Sun Jan 24, 2021 11:52 pm

Hi, I believe this applies to most monoprotic acids.


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