5022

(a) Write the chemical equation and algebraic expressions which define pK_a in terms of molar concentrations present when an acid, HA, dissociates in water.

(b) Methanol has acid-base properties that are similar to those of water. Write chemical equations which illustrate methanol functioning as an acid and as a base.

5027

Equal volumes of 1.0 molar solutions of malononitrile [(NC)₂CH₂, pK_a 11.2] and the sodium salt of phenol [C₆H₅O^–Na⁺] in water are mixed. [Phenol, C₆H₅OH, has a pK_a 10.0]. What is the equation for the reaction which occurs and what is the equilibrium constant at room temperature for that reaction? Qualitatively, which species will be present in higher concentrations at equilibrium, and which will be present in lower concentrations?

5142

Most carboxylic acids dissolve in 0.1 M sodium bicarbonate solution, but most phenols do not. This difference forms the basis for an important separation procedure. Given that

Reaction	~K_a
	10^–5
	10^–10
	10^–7

show, by calculation, why the solubilities are as stated above.

5174

Using the pK_a values given, decide whether the equilibrium will lie toward the right or the left. Give a justification for your choice.

(a)

pKa 6.5 5.2

(b)

pKa 4.3 10.2

5217

Given the pK_a of ammonium ion (9.2) and of hydrogen cyanide (9.1) shown below, calculate the free energy in kilocalories or kilojoules for the reaction.

Which species will predominate at equilibrium in a 0.1 M solution of ammonium cyanide?

5242

Using the data below, account for the fact that p-aminobenzoic acid does not exist appreciably as the dipolar ion, but p-aminobenzenesulfonic acid does.

Acid	pK_a
C₂H₅NH₃⁺	9.5
C₆H₅CO₂H	4.2
C₆H₅NH₃⁺	4.6
C₆H₅SO₃H	0.7