BR Chem Question- Acidity

[sacha]

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Why does a stronger acid produce more conjugate base when added to water? doesn't a stronger acid, have a weaker conjugate base & thus dissociates less?

BR Passages Qu 35.pg.325 (Buffers & Titration)
Given that H3CCO2H is a stronger acid than CH3NH3+, which of the following is not true?

choice c. H3CCO2H produces more conjugate base than CH3NH3+ when added to water

C is considered true, but I don't get why... i feel that it should produce less conjugate base since it dissociates less? right?

 

[michaelfranklin]

A stronger acid produces more of its conjugate base, relative to a weaker acid. This can be seen from the equation: Ka = [H+][A-] / [HA]. A stronger acid should be expected to have a higher Ka (lower pKa), and therefore a higher concentration of its conjugate base [A-].

However, the strong acid's conjugate base is expected to be weak due to the equation: Kw = 10^-14= Ka X Kb. If the Ka is higher (strong acid), Kb should be lower (weak base)

I think you are getting confused between the strength of and acid and its conjugate base, how much conjugate base is produced when comparing two different acids.

 

[cheesier]

Just to add to what michaelfranklin said, an acid is strong because it dissociates more than a weak acid. The ease from which HA --> H+ + A- determines if it's a weak acid or a strong acid. For a strong acid, you'll get more H+ and A- and less HA will remain.

You're right that a stronger acid has a weaker conjugate base, and this means that the base (A-) is less likely to become protonated.

 

[sacha]

Just to add to what michaelfranklin said, an acid is strong because it dissociates more than a weak acid. The ease from which HA --> H+ + A- determines if it's a weak acid or a strong acid. For a strong acid, you'll get more H+ and A- and less HA will remain.

You're right that a stronger acid has a weaker conjugate base, and this means that the base (A-) is less likely to become protonated.

THANK YOU both! i just got my mistake. yeah, i got confused with strength & conjugates... thanks for the response🙂