Acid dissociation in water

[Pose]

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Hi,

I have this problem:

Which of the following acids dissociate most when added to water?

A: HNO3
B: HNO2
C: H3PO4
D: H2CO3

The answer is A. But how do you know this without just memorizing pKa or Ka values of acids?

 

[NuttyEngDude]

These questions are almost always testing if you have memorized the list of strong acids/bases which will dissociate 100% (for the purposes of the mcat) in water as compared to everything else which will dissociate less. Knowing this you do not need to memorize a list of pKa or Ka values.

 

[Postictal Raiden]

Hi,

I have this problem:

Which of the following acids dissociate most when added to water?

A: HNO3
B: HNO2
C: H3PO4
D: H2CO3

The answer is A. But how do you know this without just memorizing pKa or Ka values of acids?

Strong acids dissociate almost completely in water. They are HI, HBr, HCl, HClO4, HNO3, and H2SO4. They are few. Make yourself a favor and memorize them. All other acids are considered weak, unless they are compared against other acids, then they can be "relatively strong".

 

[amberkre]

If you don't want to just memorize, the answer can be explained by the fact that HNO3 + H20 --> NO3- + H3O

If you draw out the mechanism of the acid dissociation, you find that NO3- is a very stable structure since the (-) charge is dissociated through resonance. You can draw three resonance structures for NO3- in which the three oxygen atoms each carry 2/3 of the negative charge. Since the conjugate base is very stable (making it a very weak base), you know that the acid is a very strong acid. Basically it is favorable for the H+ to leave the acid in water, so that the molecule can become a stable base.