Organic Chemistry 2 · Amine basicity and electronic structure
Compute amine speciation
Conditions: Given teaching value pKa(BH⁺) = 9.0 in water; buffer fixes pH = 8.0. Ignore activity corrections.
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The question
What fraction of total B + BH⁺ is BH⁺?
Reveal the answer and explanation
Answer and explanation
Correct answer 10/11, approximately 91%.
[B]/[BH⁺] = 10⁻¹; one part B and ten parts BH⁺ total eleven.
Not this one 1/10, or 10%.
This confuses the unprotonated-to-protonated ratio with the protonated fraction.
Not this one 1/2, or 50%.
Equal populations require pH = pKa, not a one-unit difference.
Worked solution
Henderson–Hasselbalch gives [B]/[BH⁺] = 10^(8−9) = 0.1. Set BH⁺ = 10 and B = 1; the protonated fraction is 10/(10+1) = 10/11.
What you'll practice
Convert a pH–pKa difference into a protonated fraction.
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