Question

Alanine, h2nch(ch3)co2h, has ka = 4.5  10–3 and kb = 7.4  10–5 . which species has the highest concentration at a ph of 7.00 in h2o?

Answer 1

Answer is: the highest concentration has ⁺H₃NCH(CH₃)CO₂⁻.
pKa(carboxylic group) = -log(4,5·10⁻³) = 2,35.
pKb(amino group) = -log(7,4·10⁻⁵) = 4,13.
pKa + pKb = 14.
pKa(amino group) = 14 - 4,13 = 9,87.
At pH = 7 carboxylic group lost proton(H⁺) and became COO⁻, amine is protoneted (NH₃⁺) in this pH value.