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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Astrokel on April 23, 2008, 10:03:27 AM

Title: Why strong acid has weak conjugate base?
Post by: Astrokel on April 23, 2008, 10:03:27 AM

I understand that strong/weak acid has weak/strong conjugate base.

I'm just wondering why is that so?

My teacher explained to me through following argument.

If HCOOH is a strong acid, then HCOOH ---> HCOO- + H+

HCOOH will dissociate completely.

and if HCOO- is a strong base, then it will react with the H+ ions to form back HCOOH.

This apparently contradict each other, thus strong acid must have a weak conjugate base or vice versa.

I've no problem in following the arguments, and is just wondering any molecular explanations?

Thanks,

Kelvin  ;D

Title: Re: Why strong acid has weak conjugate base?
Post by: p3t3r1 on April 23, 2008, 05:22:54 PM

If it is a strong acid, it wants to lose proton really really bad. Therefore, its conjugate base won't be able to grab on proton well and therefore will be weak. (If its conjugate base can grab on to the proton well then there wouldn't be a 100% dissociation in the first place. It would like to stay in its acidic HA form)

Title: Re: Why strong acid has weak conjugate base?
Post by: Astrokel on April 24, 2008, 08:55:51 AM

thanks!  ;)

Title: Re: Why strong acid has weak conjugate base?
Post by: AWK on April 24, 2008, 09:01:22 AM

Ka x Kb = Kw