The triprotic acid has ,

CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases

Problem

The triprotic acid H3XH_3X has pKa1=2.1pK_{a1}=2.1, pKa2=7.5pK_{a2}=7.5, and pKa3=12.0pK_{a3}=12.0. In a particular solution, 85\% of the species are HX2−HX^{2-} and 15\% are X3−X^{3-}, with all other forms negligible. Which pH value or range is most consistent with these data? (a) pH  ≈12.0pH\;\approx 12.0 (equal to pKa3pK_{a3}) (b) pH  ≈12.8pH\;\approx 12.8 (slightly above pKa3pK_{a3}) (c) pH  ≈11.2pH\;\approx 11.2 (slightly below pKa3pK_{a3}) (d) pH  ≈10.3pH\;\approx 10.3 (well below pKa3pK_{a3})

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