TBR Gen Chem Buffers question 54

[CapCrunch]

The question asks for the complete titration curve for phosphoric acid, H3PO4.

The explanation says that the first hydrogen of phosphoric acid is weak, hence answer choice B with their so-called lip-o-weakness. However, I was under the assumption that the first proton lost has a lower pKa and is thus more acidic, leading me to choose D. Can anybody straighten me out on this one? Thanks.

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[Cmdr_Shepard]

Well phosphoric acid's first proton is a weak acid in that it doesn't ionize 100% to give H + and the conjugate base (containing the two remaining protons). The second and third protons are also weak acids, but the first deprotonation has the lowest pKa.