pKa > 1 or pKa < 1 ?

[Smooth Operater]

I got this from kaplan blue book"

for weak acid, Ka < 1 since [HA] > [H+][A-]. If Ka < 1, then pKa > 1.

but I thought you have more dissociated H+ in weak acid than HA. You need to have H+ in order to make acdic solution According to Ka = [H+][A-]/[HA], Ka would be greater than 1 since [H+] > [HA]. Am I making the wrong assumption somewhere? Thanks

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[grifgin]

simply:

STRONG ACID: SMALL PKA, LARGE KA
Weak acid: Huge PKa, Small Ka

Also STRONG acid = dissociate big time
weak acid = some of molecules will dissociate, some wont

 

[Smooth Operater]

thanks grifgin, but how do you define small Ka? do you consider Ka < 1 small and Ka > 1 big?

Which lead to, how do you define big or small pKA in term of number as a reference?

I am guessing there's a number range that is considered weak acid. If so, do you know?

Thanks for the big help!

 

[grifgin]

Pka of a strong acid ie HCL ~1
Im not sure on this, but the realy really strong ones go in the negative pka

do we have a A/B expert here?

 

[djeffreyt]

I don't know if this will help but you said:

"I thought you have more dissociated H+ in weak acid than HA. You need to have H+ in order to make acdic solution "

But this is partially wrong. In a weak acid (meaning Ka < 1, and pKa > 1) you would have less dissociated H+ than HA, because weak acids do not dissociate completely in aqueous solutions, but STRONG acids completely dissociate.

I'm not sure that there is a further rule on determining whether an acid is strong or weak except to use the pKa greater or lesser than 1 thing. I would just go with that rule and not worry too much about a range, and just memorize the 4 or 5really strong acids that they reuse over and over

hydrogen sulfate
hrydrogen chloride
hydrogen fluoride
hydrogen bromide
hydrogen phosphate

I'm sure there is one or two more I'm missing, but off the top of my head, that's what I recall.

 

[SgtSadhu]

Hey i think a good way of looking at this questions is that

ka= [h+][a-]
[ha]

If the acid is weak it won't dissociate alot so the numerator will be less than the denominator which is the original acid

if you assume at equalibrium the denominator and numerator are equal, then you see that if the denominator is more, it will be less than 1