Question about acidity and negatively charged product

[DrMantisTobogganMD]

Just took the 2015 practice test from AAMC and was asked the following question...

Each of the following equations shows the dissociation of an acid in water. Which of the reactions occurs to the LEAST extent?

HCl + H2O --> H3O+ + Cl-

HPO4 2- + H2O --> H3O+ + PO4 3-

H2SO4 + H2O --> H3O+ + HSO4-

H3PO4 + H2O --> H3O+ + H2PO4-

The second equation was the correct answer. Is it the least likely to occur because the product is so negatively charged and can't handle it?

Any help is really appreciated, thanks!

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[memory tcells]

So my Chemistry is rusty because I took the MCAT 2 years ago but this question essentially asks which acid is the weakest (least likely to give up Hydrogen ion). Eq.1 and 3 are out because they are strong acids so that narrows it down to #2 and #4. HPO4 and H3PO4 are similiar because they both have (PO4) but since #4 has more protons, it is more likely to give one up than #2.

At least that's how I would approach this problem. Someone who has taken Chemistry or the MCAT recently can probably explain it better in scientific terms.

 

[aldol16]

Yeah, basically. HCl aand H2SO4 you should know to be strong acids. So it's down to H3PO4 and HPO4 - since the pKa's of multi-protic acids increase as you lose more protons, you know that the later deprotonations are less favorable than the initial deprotonations. Also for reasons of charge here as well.

 

[DrMantisTobogganMD]

That's a good way a to approach it, appreciate the help.

 

[memory tcells]

Yeah, basically. HCl aand H2SO4 you should know to be strong acids. So it's down to H3PO4 and HPO4 - since the pKa's of multi-protic acids increase as you lose more protons, you know that the later deprotonations are less favorable than the initial deprotonations. Also for reasons of charge here as well.

Yup, this is how I wanted to sound. Instead I had word vomit.