Question

For the amino acid alanine, the major species in solution at pH 7 is the zwitterionic form, which has a negatively charged carboxylic acid group and a positively charged amino group. There is a less common neutral form in which neither group is charged. The carboxylic acid group of alanine has a p K a of 3 . The amino group of alanine has a p K a of 8 . Estimate the ratio of the concentration of the neutral amino acid species to the zwitterionic species at pH 7 .

Answer 1

Answer: ratio is 10⁻⁵

Step-by-step explanation:

From Acid-dissociation equilibrium constant, HA is given thus,

HA + H₂O ⇄ H₃O⁺ + A⁻

equilibrium constant, Ka = [H₃O⁺] [A⁻] / [HA]

[H₃O⁺] = Ka [HA] / [A⁻]

from this we have that

pH = -log[H₃O⁺]

and pKa = -logKa

Applying Henderson-Hassel bach equation;

pH = pKa + log [A⁻]/[HA] ...........(1)

A⁻ rep the salt and HA rep the acid

at a given pH of 7, the pKa value of carboxylic acid COOH is 3

∴ from equation (1).

7 = 3 + log(COO⁻/COOH)

log(COO⁻/COOH) = 4

thus (COO⁻/COOH) = 10⁻⁴ ...................(2)

The ratio of alanine COOH to COO- at pH 7 is (COO⁻/COOH) = 10⁻⁴

(ii) Also considering the ratio of alanine NH2 to NH3+ at pH 7 we have;

given Pka value is 8,

therefore pH = pKa + log [salt]/[base]

7 = 8 + log (NH₂/NH₃)

(NH₂/NH₃) = 10⁻¹

thus the ratio of the concentration of the neutral amino acid species to the zwitterionic species at pH 7 is;

10⁻⁴ ˣ 10⁻¹ = 10⁻⁵

cheers i hope this helps