AAMC #3, Question 35

[discowisco]

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Can someone explain to me how Oxalic acid contains a greater molar concentration of solute particles than propionic acid. I understand that oxalic is a diprotic, is that the reason why?

The explaination says something aout oxalic avid requiring more NaOH than does crotonic acid to reach a ph of 4.7? How is that?

I undertand that if ph > pka then the substance is deprotonated so a pH of 4.7 would protonate 1 of oxalic acids H+ and the only H of crotnic acid. Thus dont both give up only one H+ to solution?

 

[deh2ase]

Remember that the more NaOH (solute) that is added, the lower the freezing point (because of freezing point depression). You are looking for the acid that had more NaOH titrated. For crotonic acid the pKa is 4.7, remember that if pH = pKa, then only half of the crotonic acid molecules have been deprotonated by the base (look back at the henderson-hesselbach equation). For oxalic acid, the pKa of the first proton is 3.1. Since the pH is well above pKa, all of the first proton from oxalic acid has been deprotonated. Which is why it requires more NaOH to pH oxalic acid to 4.7.

 

[discowisco]

The pka of crotonic acid is 4.69, isnt 4.7 > 4.69 thus shouldnt that be deprotonated too?

 

[deh2ase]

Remember henderson-hesselbach, states pH = pKa + log ([conjugate acid] / [acid])

If you plug in the values for crotonic acid into henderson-hesselbach, the (conjugate acid / acid) ratio is 10^0.01 which is still very close to 1 so only half the acid has been deprotonated by the base. If you plug in the values for oxalic acid, the (conjugate acid / acid) ratio is 10^1.6. For the MCAT, you can just compare 10^0.01 vs. 10^1.6 to see which value is bigger and that tells you which one formed more conjugate acid (from more NaOH being added).

 

[Jepstein30]

The pka of crotonic acid is 4.69, isnt 4.7 > 4.69 thus shouldnt that be deprotonated too?

They will both be deprotonated but oxalic acid will require twice as much base to deprotonate it because it is diprotic.

regardless, C is the ONLY answer that makes sense here. Higher BP = higher solute concentration. The other answers don't relate colligative properties properly.