A naturally occurring amino acid such as alanine has a group that is a carboxylic acid and a group that is a protonated amine. The pK_a values of the two groups are shown.

c. Draw the structure of alanine in a solution at physiological pH (pH 7.4).

Calculate the equilibrium constant for the acid–base reaction between the reactants in each of the following pairs:

c. CH₃NH₂ + H₂O

d. CH₃N⁺H₃ + H2O

Which reaction in Problem 23 has the smallest equilibrium constant?

Given that pH + pOH = 14 and that the concentration of water in a solution of water is 55.5 M, show that the pK_a of water is 15.7. (Hint: pOH=−log [HO−])

For each of the following compounds, draw the form that predominates at pH = 3, pH = 6, pH = 10, and pH = 14:

b. CH₃CH₂N⁺H₃ (pK_a = 11.0)

For each of the following compounds (here shown in their acidic forms), write the form that ­predominates in a solution with a pH = 5.5:

a. CH₃COOH(pK_a = 4.76)

b. CH₃CH₂N⁺H₃ (pK_a = 11.0)

At what pH is the concentration of a compound, with a pK_a = 8.4, 100 times greater in its acidic form than in its basic form? At what pH is 50% of a compound, with a pK_a = 7.3, in its basic form?

If an acid with a pK_a of 5.3 is in an aqueous solution of pH 5.7, what percentage of the acid is present in its acidic form?

For each of the following compounds (here shown in their acidic forms), write the form that ­predominates in a solution with a pH = 5.5:

i. HON⁺H₃ (pK_a = 6.0)