(10 pts total)

The amino acid alanine has the following structure which we could abbreviate as AlaH₂⁺:

K1 = 4.49 X 10-3 K2 = 1.36 X 10-10		AlaH2+ AlaH Ala-

What are the concentrations of the different acid/base species of alanine present in a 0.05 M solution at pH 5? (Show all work.)

K₁ = 4.49 X 10^-3 = [AlaH][H⁺]/[AlaH₂⁺] --> [AlaH]/[AlaH₂⁺] = (4.49 X 10^-3)/(10^-5 ) = 4.49 X 10² ; that is, [AlaH]>>[AlaH₂⁺] (2 pts)

K₂ = 1.36 X 10^-10 = [Ala^-][H⁺]/[AlaH] --> [Ala^-]/[AlaH] = (1.36 X 10^-10)/(10^-5 ) = 1.36 X 10^-5; that is, [AlaH] >>[Ala^-] (2 pts)

[AlaH₂⁺] + [AlaH] + [Ala^-] = 0.05M ~ [AlaH] (2 pts)

[AlaH₂⁺] = [AlaH]/4.49 X 10² = 0.05/4.49 X 10² = 1.11 X 10^-4 M (2 pts)

[Ala^-] = [AlaH](1.36 X 10^-5) = (0.05)(1.36 X 10^-5) = 6.75 X 10^-7 M (2 pts)