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Postby ursulavictorino1K » Fri Jan 24, 2020 12:52 am

Why is it that a value of k<10^-3 means it is a weak acid?

Bryan Chen 1H
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Re: k<10^-3

Postby Bryan Chen 1H » Fri Jan 24, 2020 6:18 am

there's no set number that makes something definitely "weak" or "strong" i'd say but compared to strong acids like HCl, an acid that only dissociates 1 proton per 1000 other molecules is much weaker relatively speaking.

Sean Cheah 1E
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Re: k<10^-3

Postby Sean Cheah 1E » Fri Jan 24, 2020 9:00 am

I don't believe that there is a strict cutoff of k = 10^-3 when determining whether or not a given compound is a weak acid. An acid is considered weak if it does not completely ionize in solution. That means that even acids with a Ka of 1 can technically be considered weak because this would imply that the concentration of ionized is equal to the concentration unionized species, which could only happen in the case of incomplete ionization.

Sanjana Munagala_1j
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Re: k<10^-3

Postby Sanjana Munagala_1j » Fri Jan 24, 2020 8:17 pm

The general idea is that for a weak acid, the Ka value is going to be very small. Let's say you have this reaction for an acid in water:
HA(aq) + H2O --> H3O+(aq) + A-(aq)

The Ka for this reaction would be:


Since the reaction is of a weak acid, it won't fully dissociate meaning it would produce lesser quantities of the products, which makes the numerator in the Ka equation small and thus the whole Ka for a weak acid reaction to be small.

Hope that helps!

Bella Townsend
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Re: k<10^-3

Postby Bella Townsend » Fri Jan 24, 2020 8:23 pm

the scale is relative. the smaller the K value the stronger the acid.

Emil Velasco 1H
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Re: k<10^-3

Postby Emil Velasco 1H » Fri Jan 24, 2020 9:33 pm

I think it's just a given concept but not necessarily set in stone for identification purposes

William Francis 2E
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Re: k<10^-3

Postby William Francis 2E » Fri Jan 24, 2020 10:22 pm

I think acids can still be considered weak with a Ka higher than 10^-3. Strong acids almost completely dissociate in water, so Ka values are never even really given for them since they would be so large. 10^-3 has (somewhat arbitrarily) been deemed the cutoff point to distinguish small K values from not-small K values for weak acids. There's no complex mathematical proof to justify this that I am aware of; chemists just set that as the cutoff point. If the Ka is less than 10^-3, you can usually assume that the concentration of hydronium ions in a solution where the acid has been added to water will be negligibly small compared to the initial concentration of the acid, which makes calculations easier.

Kylie Lim 4G
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Re: k<10^-3

Postby Kylie Lim 4G » Fri Jan 24, 2020 11:52 pm

k<10^-3 is a weak acid because to have a small number, there has to be a high concentration of reactants. If there are a lot of reactants, the acid is weak because it does not completely dissociate.

Megan Cao 1I
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Re: k<10^-3

Postby Megan Cao 1I » Sat Jan 25, 2020 12:22 am

k<10-3 helps us know it's a weak acid because in the equalibrium constant, in order for it to be such a small number the amount of reactants must be greater than the products. thus telling us that the acid barely dissociates, making it weak by definition

andrewcj 2C
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Re: k<10^-3

Postby andrewcj 2C » Sat Jan 25, 2020 1:34 am

Something to remember is that strong acids don't have K values. In the k expression, the denominator would be essentailly 0, which is an invalid expression.

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