Comparing Polyprotic Pka

[discowisco]

Which of the following relationships must be true?

I. Pka1 (h3po4) is less than pka (HF)

II. PKa2 (H3PO4) is less than pka2 (H2SO4)

II. Pka (HF) is less than Pka1 (H2SO4)


what information do i need to know(or have memorized) to tackle this problem?

Answer is I only

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[krogers21]

Which of the following relationships must be true?

I. Pka1 (h3po4) is less than pka (HF)

II. PKa2 (H3PO4) is less than pka2 (H2SO4)

II. Pka (HF) is less than Pka1 (H2SO4)


what information do i need to know(or have memorized) to tackle this problem?

Answer is I only

Good question! Starting with III, pKa1 of H2SO4 has to be lower than the pKa of HF because h2so4 is a strong acid. Similarly, pka2 is lower in a strong acid than pka2 of a weak acid so we can eliminate choice 2. By default, it can only be the choice with I. I think it may be possible to reason out why the pka1 of h3po4 is less than the pKa of HF due to it being triprotic, but I personally do not know so I would use testing logic to eliminate the wrong answers. I think that sometimes the MCAT tests us not on whether an answer is right but on whether the other answers are wrong. Knowing this is probably the best advice I could ever give for the MCAT.