Soal IChO Ke-26

THEORETICAL PROBLEM No. 3

(26.5 blue points, 9 red points)

Sulphur forms many different compounds with oxygen and halogens (sulphur as the central atom). These compounds are mainly molecular, and many are easily hydrolysed in water.

a) Write Lewis structures for the molecules SCl₂, SO₃, SO₂ClF, SF₄, and SBrF₅.

b) Carefully draw the geometries of the same 5 molecules. (Disregard small deviations from “ideal” angles.)

c) A compound, consisting of sulphur (one atom per molecule), oxygen and one or more of the elements F, Cl, Br, and I, was examined. A small amount of the substance reacted with water. It was completely hydrolyzed without any oxidation or reduction, and all reaction products dissolved. 0.1 M solutions of a series of test reagents were added to separate, small portions of a diluted solution of the substance.

Which ions are being tested for in the following tests:

i) Addition of HNO₃ and AgNO₃ .

ii) Addition of Ba(NO₃)₂.

iii) Adjustment to pH = 7 with NH₃ and addition of Ca(NO₃)₂.

Write the equations for the possible reactions in the tests:

iv) Addition of KMnO₄ followed by Ba(NO₃)₂ to an acid solution of the substance.

v) Addition of Cu(NO₃)₂.

d) In practice, the tests in c) gave the following results:

i) A yellowish precipitate

ii) No precipitate

iii) No visible reaction

iv) The main features were that the characteristic colour of permanganate disappeared, and a white precipitate was formed upon addition of Ba(NO₃)₂

v) No precipitate

Write the formulas of the possible compounds, taking the results of these tests into account.

e) Finally, a simple quantitative analysis was undertaken:

7.190 g of the substance was weighed out and dissolved in water to give 250.0 cm³ solution. To 25.00 cm³ of this solution, nitric acid, and enough AgNO₃to secure complete precipitation, was added. The precipitate weighed 1.452 g after washing and drying.

Determine the formula of the compound.

f) Write the equation describing the reaction of the substance with water.

If you have not arrived at a formula for the compound, use SOClF.

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