ACJC 2015_H1_CHEM_P1_Answers Prelims

© ACJC 2015 8872/01/Prelim/15 [Turn over 2 Section A For each question there are four possible answers, A, B, C, and D. Choose the one you consider to be correct. 1 Tetraarsenic trisulfide, As4S3 is used as a maintenance therapy for newly diagnosed acute leukemia. It reacts with oxygen to produce an oxide of arsenic in the +5 oxidation state and an oxide of sulfur in the +4 oxidation state. How many moles of oxygen gas are needed to burn two moles of As4S3? A 7.5 B 8 C 15 D 16 2 In an experiment, 33.40 cm3 of a 0.150 mol dm−3 solution of a metallic ion, Mn+, reacted exactly with 50.0 cm3 of 0.050 mol dm−3 aqueous sodium sulfite. The half-equation for the oxidation of sulfite ion is shown below. SO3 2− (aq) + H2O (l)  SO4 2− (aq) + 2H+ (aq) + 2e− If the final oxidation number of the metal in the salt was +3, what would be the original oxidation number of the metal? A +2 B +3 C +4 D +5 3 Titanium has the electronic structure, 1s2 2s2 2p6 3s2 3p6 3d2 4s2. Which one of the following compound is NOT likely to exist? A TiN B KTiO4 C TiS D Na3TiF6

© ACJC 2015 8872/01/Prelim/15 [Turn over 3 4 The successive ionisation energies, in kJ mol –1, of an element X are given below. 1010 2265 3662 4998 7002 8992 28698 32215 Which one of the following would best represent the formula of the ionic compound formed by X? A XBr B XO2 C MgX2 D K2X 5 Which molecule has the largest dipole? A B C D 6 Which of the following pairs of substances have different types of bonding and

© ACJC 2015 8872/01/Prelim/15 [Turn over 4 structure? A AsCl3 AsCl5 B AlF3 AlCl3 C P4 PH3 D Si SiO2 7 Which of the following cannot be explained by hydrogen bonding? A Ethanoic acid molecules form dimers in presence of benzene. B Boiling point of alcohols increases with increasing Mr. C Ice can float on frozen lake. D Formation of HF2 ¯. 8 A student weighed 4.20 g of solid sodium hydrogencarbonate, NaHCO 3 (Mr = 84.0), and dissolved it completely in 5 0 cm3 of water. The drop in temperature was 3.8 oC. What is the enthalpy change of this reaction in kJ mol-1? (The specific heat capacity of solution is 4.18 J cm-3 oC-1) A 1000 3.8 x 4.18 x 4.2      B 1000 x 4.2 84.0 x 3.8 x 4.18 x 50       C 1000 3.8 x 4.18 x 50      D 1000 x 4.2 84.0 x 3.8 x 4.18 x 4.2      

© ACJC 2015 8872/01/Prelim/15 [Turn over 5 9 The kinetics of the reaction between thiosulfate ions and acid is studied by varying the concentrations and measuring the time taken for the ‘cross’ to be totally obscured by the sulfur produced. The results are recorded in the table below. S2O3 2-(aq) + 2H+(aq)  H2O(l) +SO2(g) + S(s) Experiment Vol of S2O3 2- /cm3 Vol of H+/cm3 Vol of H2O/cm3 Time taken/min 1 30 30 10 11.0 2 30 40 0 10.0 3 45 20 5 6.5 What is the order of reaction with respect to ea