International Chemistry Olympiad · 1998
Problems
- Problem 1A 0.4062 g alloy sample containing tin and lead is dissolved in hot / (Pb → Pb(II), Sn → Sn(IV)); on cooling a precipitate of tin compounds and a lead compound appears. of 0.2000 M is added (the precipitate dissolves) and the solution is diluted to . A aliquot, buffered at pH 6 with hexamine and Xylenol Orange indicator, is titrated with standard 0.009970 M : the colour changes yellow→red at . Then 2.0 g of solid is added — the solution returns to yellow — and titration resumes to a second endpoint at . ( binds strongly but not Pb(II) at pH 6.) (1.1) What lead compound precipitates in step 2? (1.3–1.8) Give the roles of hexamine, Xylenol Orange and , and the key ionic equations of the two titrations. (1.10) Calculate the weight percentages of Sn and Pb in the alloy.Solutions: 1
- Problem 2Dating historical events using . Nathan Thompson, an early settler of Lord Howe Island, planted European deciduous trees; the year is unknown. Pollen of European oak and elm (which pollinate in their first year) accumulated in lake sediment together with radioactive (half-life = 22.0 years). In 1995 a sediment core was taken: oak/elm pollen first occurs at 50 cm depth, where the activity is 1.40 Bq kg⁻¹, versus 356 Bq kg⁻¹ at the top of the core. (2.1) In what year were the seeds planted? (2.2) is a daughter of , which stays in the earth's crust: (stable). Which step explains how reaches rainwater while its parent remains in the crust?Solutions: 1