Chemistry Labs

International Chemistry Olympiad · 2024

Problems

  1. Problem 1The element gallium (Ga, Z=31Z=31) is recovered as a by-product of aluminium production. Gallium nitride is made by heating GaX2OX3\ce{Ga2O3} with ammonia: GaX2OX3+2 NHX3→2 GaN+3 HX2O\ce{Ga2O3 + 2NH3 -> 2GaN + 3H2O}. Calculate the mass of GaX2OX3\ce{Ga2O3} needed to prepare 100 g of GaN. (M(GaX2OX3)=187.4M(\ce{Ga2O3}) = 187.4 g mol−1^{-1}, M(GaN)=83.7M(\ce{GaN}) = 83.7 g mol−1^{-1})Solutions: 1
  2. Problem 2Resistive gas sensors using semiconducting metal oxides (SMOX) detect tiny amounts of impurities. The mixed oxide X, which has the normal spinel structure A2+B23+O42−\mathrm{A^{2+}B^{3+}_2O^{2-}_4}, is obtained by decomposing in air the crystalline hydrated metal oxalate ZCX2OX4 ⋅ kHX2O\ce{ZC2O4*\mathit{k}H2O} of metal Z. When heated to 140 °C the hydrate loses 19.7 % of its mass; further heating in air at 500 °C yields 2.407 g of black X together with 3.8 dm3^3 of COX2\ce{CO2} (at 101325 Pa, 500 °C). (a) Determine the formula of X and the value of kk. (b) In the spinel structure the OX2−\ce{O^{2-}} ions form a face-centred cubic lattice; A2+\mathrm{A^{2+}} cations occupy part of the tetrahedral voids and B3+\mathrm{B^{3+}} cations part of the octahedral voids. Calculate the percentage of tetrahedral sites occupied in a normal spinel AB2O4\mathrm{AB_2O_4}.Solutions: 1