3. Acids and Bases

For the following acid–base pairs, (iv) calculate K_eq;

(e)

Given that the indicated pK_a values correspond to the acid dissociation reactions shown, calculate the ratio of acid to conjugate base for the reactions shown.

(a)

Using pK_a values, calculate an approximate K_eq value for the following substitution reaction.

At what pH is the concentration of a compound, with a pK_a = 4.6, 10 times greater in its basic form than in its acidic form?

At what pH does 80% of the acid exist in its acidic form?

The pK_a of a protonated alcohol is about -2.5, and the pK_a of an alcohol is about 15. Therefore, as long as the pH of the solution is greater than _______ and less than _______, more than 50% of 2-propanol (the product of the reaction on p. 244) will be in its neutral, nonprotonated form.

The pK_a of protonated acetone is about –7.5 and the pK_a of protonated hydroxylamine is 6.0.

a. In a reaction with hydroxylamine at pH 4.5 (Figure 16.2), what fraction of acetone is present in its acidic, protonated form?

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The pK_a of acetic acid in water is 4.76. What effect will a decrease in the polarity of the solvent have on the pK_a? Why?

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)

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).

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?