International Chemistry Olympiad · 1988

Problems

Problem 2Heating a mixture of element A and fluorine (molar ratio 1 : 9, pressure ~1 MPa) to 900 °C gives three crystalline products B, C, D (all melting below 150 °C). The fluorine content of C is 36.7 % and of D is 46.5 % (by weight). Reaction of B with anhydrous $\ce{HOSO2F}$ at -75 °C gives E + HF; X-ray diffraction maps of E show electron density maxima with values 52, 58, 104 and 350, roughly proportional to atomic numbers. 2.1 Assign these maxima to the elements in E (F, O, S, A) and determine the atomic number and identity of A. 2.2 Treating 450.0 mg of C with excess mercury liberates 53.25 cm $^3$ of gas A at 101.0 kPa and 25 °C. Calculate the relative atomic mass of A and identify B, C, D and E. 2.3 Hydrolysis in water proceeds as follows: B hydrolyses to A, $\ce{O2}$ and HF; C hydrolyses to A, $\ce{O2}$ (in 4 : 3 molar ratio), aqueous $\ce{AO3}$ and HF; D hydrolyses to aqueous $\ce{AO3}$ and HF. Write the three balanced hydrolysis equations. 2.4 Quantitative hydrolysis of a mixture of B, C, D yields 60.2 cm $^3$ of gas at 290 K and 100 kPa containing 40.0 % $\ce{O2}$ by volume. The dissolved $\ce{AO3}$ is titrated with 0.100 M $\ce{FeSO4}$ , requiring 36.0 cm $^3$ ( $\ce{Fe^{2+}}$ is oxidised to $\ce{Fe^{3+}}$ and $\ce{AO3}$ is reduced to A). Calculate the molar composition of the original mixture of B, C, D.Solutions: 1