Ex 4.4 tbr acid and base

[Meredith92]

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Im not sure how tbr arrived at this answer unless they expect us to memorize lots of pKa values

Which of the following acids dissociates to the greatest extent when added to water
A. HNO3
B HNO2
C H3PO4
D H2CO3

The answer is a
But when drawing the structures out it seems that HNO3 would not "want" to have a proton removed because it would create more charge. Should I have chosen a anyway because it has one more resonance form despite the extra charge? I'm never sure how to approach these questions :/ thanks for your help!

 

[IanMalcoRaptor]

You should have chosen A because all the others are "weak" acids. strong acids automatically dissociate completely (for all intents and purposes) in water, especially compared to weak acids.

In fact, I believe that is how strong acids are defined...they dissolve completely in water

 

[Meredith92]

But did you know hno3 was a strong acid just from memorizing it? If I didn't have that memorized how would I know hno3 is a strong acid since losing a proton makes it carry more charge?

 

[gettheleadout]

But did you know hno3 was a strong acid just from memorizing it? If I didn't have that memorized how would I know hno3 is a strong acid since losing a proton makes it carry more charge?

1) You should absolutely memorize the strong acids: HNO3, H2SO4, HClO4, HClO3, HBr, HCl, and HI.

2) The negative charge gained upon deprotonation is stabilized via resonance within the nitrate anion, and is quite stable in aqueous solution. Further, many, many acids (all of the strong ones) gain a negative charge upon ionization (hence the term "ionization"). The only example I can think of off the top of my head that instead becomes neutral is ammonium (NH4+), and ammonium is a weak acid.

 

[Meredith92]

Okay thanks that makes sense

I just feel like I'm abandoning my organic chemistry knowledge by accepting that having a positive charge on N ( which is electronegative) and having TWO negative charges is a good structure. For HNO3 to be a strong acid it's conjugate base should be stable/ have a decent structure... And it's hard for me to easily grasp why this works well. I guess the additional resonance structure makes up for these bad charge placements

 

[IanMalcoRaptor]

Resonance stability trumps

edit:
and yeah those 6 acids aren't too much to memorize, but you should have from gen II and/or organic