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, pKa 11.2] and
the sodium salt of phenol [C6H5O–Na+] in water are mixed. [Phenol, C6H5OH,
has a pKa 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
RCO2H
+
ArCO2H
H2CO3
+
+
H2O
H2O
H2O
~Ka
RCO2
+
H3O+
10–5
ArCO2
+
H3O+
10–10
HCO3
+
H3O+
10–7
show, by calculation, why the solubilities are as stated above.
5174
Using the pKa values given, decide whether the equilibrium will lie toward the
right or the left. Give a justification for your choice.
(a)
–
N+H + HCO3
+ H2CO3
N
pKa
6.5
5.2
(b)
O
O
H3C
pKa
C
4.3
OH + –CH2NO2
H3C
C
O– + CH3NO2
10.2
5217
Given the pKa of ammonium ion (9.2) and of hydrogen cyanide (9.1) shown
below, calculate the free energy in kilocalories or kilojoules for the reaction.
NH4+ + CN–
NH3 + HCN
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
+
C2H5NH3
C6H5CO2H
C6H5NH3+
C6H5SO3H
pKa
9.5
4.2
4.6
0.7