Tutorial 3 - Chemical Equilibria
Question 1
Calculate the solubility of Fe(OH)3 in water using the systematic approach to chemical equilibria.
Ksp = 4.010-38.
Question 2
(a) Use the systematic treatment of equilibrium to calculate the concentration of Zn 2+ (mg.L-1) in
a saturated solution of Zn2[Fe(CN)6], which dissociates into Zn2+ and Fe(CN)64(ferrocyanide). Ksp = 2.110-16.
(b) Calculate the molar concentration of ferrocyanide in a solution of 0.040 M ZnSO 4 saturated
with Zn2[Fe(CN)6]. Note that ZnSO4 will be completely dissociated into Zn2+ and SO42-.
(c) What molar concentration of K4Fe(CN)6 should be added to a suspension of solid
Zn2Fe(CN)6 in water to give [Zn2+] = 5.010-7 M?
Question 3
Find [M2+] in a saturated solution of the salt MA2 in which [OH-] = 1.810-10 M. The equilibria
are:
MA2(s) M2+ + 2A-
Ksp = 6.310-14
A- + H2O  HA + OH-
Kb = 1.810-10
Question 4
When ammonium sulphate dissolves, both the anion and cation have acid-base reactions:
(NH4)2SO4(s)  2NH4+ + SO42-
Ksp = 276
NH4+  NH3(aq) + H+
Ka = 5.7010-10
SO42- + H2O  HSO4- + OH-
Kb = 9.8010-13
(a) Write a charge balance for this system.
(b) Write a mass balance for this system.
(c) Find the concentration of NH3(aq) if the pH is fixed at 9.25.
Question 5
Find the ionic strength of:
(a) 0.02 M Na2SO4
(b) 0.02 M Fe2(SO4)3
Question 6
Calculate the activity of Zn2+ (ionic size = 600 pm) and SO42- (ionic size = 400 pm) ions in a
0.200 mol/L solution of ZnSO4.
Question 7
Use activities to calculate the solubility of Ba(IO3)2 in a 0.033 M solution of Mg(IO3)2. Ksp for
Ba(IO3)2 is 1.5710-9. Assume that the Ba2+ and IO3- from the precipitate do not significantly
affect the ionic strength of the solution since the value of Ksp is very small.
Additional Questions
Question 8
Using activities, calculate the hydronium ion concentration in a 0.0500 M solution of HNO2 that
is also 0.1200 M in NaCl. Ka = 5.110-4.
Question 9
Find the concentration of Cu2+ in a solution saturated with Cu4(OH)6(SO4), if [OH-] is somehow
fixed at 1.010-6 M.
Cu4(OH)6(SO4)(s)  4Cu2+ + 6OH- + SO42-
Ksp = 2.310-69