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 each of the reactant? Order with respect to S2O3 2- Order with respect to H+ A 0 1 B 1 0 C 1 1 D 2 0

© ACJC 2015 8872/01/Prelim/15 [Turn over 6 10 The reaction of hydrogen peroxide with iodide ions in acidic medium is as shown: H2O2(aq) + 2H+(aq) + 2Iˉ(aq)  2H2O(l) + I2(aq) It is found that the order of reaction with respect to hydrogen peroxide and iodide ions is 1 respectively. The product of concentrations of hydrogen peroxide and iodide ions is plotted against time, t for the following reaction. Which of the following graph i s obtained? A B C D

© ACJC 2015 8872/01/Prelim/15 [Turn over 7 11 W and Y are mixed in a vessel to form X as shown. At time t min, the equilibrium mixture is subjected to a change. 2W(g) + Y(g) 2X(g) ΔH < 0 Which of the following changes made would result in the graph above? A Decrease the temperature. B Decrease the volume of container. C Add 1 mol of neon gas. D Add 1 mol of X(g). 12 Water dissociates into H+ and OH– as shown. H2O H+ + OH– At 25 °C, the equilibrium [H+] is 10–7 mol dm–3; density of H2O = 1 kg dm–3. What is the order of increasing numerical value of pH, pKa and pKw for this equilibrium at this temperature? A pH, pKa, pKw B pH, pKw, pKa C pKa, pKw, pH D pKw, pKa, pH X Y W Concentration Time t

© ACJC 2015 8872/01/Prelim/15 [Turn over 8 13 An enzyme required in laboratory process operates at maximum efficiency when placed in an aqueous solution buffered at pH 5.6. Which combination of substances when dissolved in water would give the appropriate buffer solution? A 0.5 mol of HCl and 1 mol of CH3COOH B 0.5 mol of HCl and 1 mol of CH3COONa C 1 mol of CH3COOH and 1 mol of NaOH D 1 mol of CH3COONH4 14 Which factor helps to explain why the first ionisation energies of the Group II elements decrease from beryllium to magnesium to calcium to strontium? A The nuclear charge of the elements increases. B The shielding effect of the inner shells increases. C The outer electron is in ‘s’ subshell. D The repulsion between spin–paired electrons increases. 15 P, Q and R are consecutive elements in Period 3 of the Periodic Table. Q has the highest first ionisation energy and the lowest melting point among the three elements. Which of the following could be P, Q and R? A aluminium, silicon, phosphorus B sodium, magnesium, aluminium C magnesium, aluminium, silicon D silicon, phosphorus, sulfur

© ACJC 2015 8872/01/Prelim/15 [Turn over 9 16 Element S is found in Period 3 of the Periodic Table. The oxide and chloride of element S are separately mixed with wate r. The two resulting solutions have the same effect on universal indicator. What is the identity of element S? A Sodium B Magnesium C Aluminum D Phosphorous 17 Cholesterol is the most common steroid alcohol. It has a molecular formula of C27H46O and has the structure shown. How many carbon atoms are in the hydrocarbon group R? A 7 B 8 C 9 D 10

© ACJC 2015 8872/01/Prelim/15 [Turn over 10 18 Which property of benzene may be directly attributed to the stability associated with its delocalised electrons? A It has a low boiling point. B It tends to undergo substitution rather than addition reactions. C It does not conduct electricity. D It does not react with sodium. 19 Jasmone is an active ingredient of jasmine. It is extracted from jasmine flowers for perfume. Jasmone Which of the following statements is correct? A It exists as a pair of cis-trans isomers. B There are 4 sp3 carbon atoms in Jasmone. C Effervescence is observed when Jasmone is heated with acidified KMnO4. D A reddish brown precipitate is observed when hot alkaline Cu 2+ complex solution was added to Jasmone.

© ACJC 2015 8872/01/Prelim/15 [Turn over 11 20 The compound C3H7Br undergoes a sequence of reactions as follows: OH–(aq) Cr2O7 2–(aq) Tollens’ reagent/H+ C3H7Br X Y Z + silver mirror What are the compounds X, Y and Z? X Y Z A CH3CH(OH)CH3 CH3CH2CO2H CH3CH2CHO B CH3CH2CH2OH CH3CH2CHO CH3CO2H C CH3CH2CH2OH CH3CH2CHO CH3CH2CO2H D CH3CH(OH)CH3 CH3COCH3 CH3CO2H 21 Which compounds will not produce CHI3 on warming with alkaline I2(aq)? A CH3CO2CHICOCH3 B C6H5COCH2I C CH3CO2CI3 D I2CHCH(OH)CH3 22 Which of the following is arranged in order of decreasing pKa? A ClCH2CO2H , BrCH2CO2H, BrCH2CH2CO2H, CH3CH2OH B CH3CH2OH, BrCH2CO2H, BrCH2CH2CO2H, ClCH2CO2H C CH3CH2OH, BrCH2CH2CO2H, BrCH2CO2H, ClCH2CO2H D BrCH2CO2H, ClCH2CO2H, BrCH2CH2CO2H, CH3CH2OH