In this section we publish best tasks from the previous IChO in Moscow held in 1996. Solution follows text of the problem with a two weeks gap. The tasks will be changed once a month. Have a nice time!

The Best IChO tasks from 1980-1990

Word

Moscow, 1996. Inorganic chemistry

Potassium dichromate is one of the most widely used precipitating reagents. The following equilibria are established in aqueous solutions of Cr(VI).
HCrO₄^- + H₂O = CrO₄^2- + H₃O⁺ pK₁ = 6.50
2HCrO₄^- = Cr₂O₇^2- + H₂O pK₂ = -1.36
All other equlibria with involving of chromium should be ignored. All the activity coefficients are assumed to be 1.
1. The ionic product of water K_w = 1.0∙10^-14. Evaluate the equilibrium constants
a) CrO₄^2- + H₂O = HCrO₄^- + OH^-
b) Cr₂O₇^2- + 2OH^- = 2CrO₄^2- + H₂O
2. The solubility product of BaCrO₄ K_s = 1.2∙10^-10. BaCr₂O₇ is very soluble in water. In what direction will the equilibrium 1b shift upon the addition of the following reagents to a moderately concentrated aqueous solution of potassium dichromate?
a) KOH
b) HCl
c) BaCl₂
d) H₂O (consider all the above equilibria).
3. Dissociation constant of acetic acid K_a = 1.8∙10^-5. Calculate the pH value of the following solutions
a) 0.010 M K₂CrO₄
b) 0.010 M K₂Cr₂O₇
c) 0.010 M K₂Cr₂O₇ + 0.10 M CH₃COOH
4. Calculate the equilibrium concentrations of the following ions in a solution of 0.010 M K₂Cr₂O₇ + 0.10 M CH₃COOH
a) CrO₄^2-
b) Cr₂O₇^2-

Solution

Moscow, 1996. Physical chemistry

Moscow, 1996. Organic chemistry