VJC Prelim P1

2  VJC 2015 9647/01/PRELIM/15 [Turn over Section A For each question there are four possible answers, A, B, C, and D. Choose the one you consider to be correct. 1 Which one of the following statements about 20.3 g of Co2(SO4)3 is incorrect? [Molar mass = 406 g mol-1, Avogadro’s constant = 6.0 x 1023 mol-1] A It contains 0.10 mol of Co3+ ions. B It contains 0.15 mol of SO4 2- ions. C It contains 47.3% of oxygen by mass. D It contains 1.5 x 1023 atoms. 2 In theory, 3 mol of NH3OH+(aq) is required to convert 1 mol of BrO3 –(aq) to Br–(aq). Which one of the following statements about this reaction is correct? A NH3OH+ is acting as an oxidising agent. B The change in oxidation number of nitrogen is +4. C N2O is the nitrogen–containing product. D The reaction is likely to have a high activation energy. 3 Which one of the following is a possible configuration of a stable M3+ ion in the ground state? A 1s2 2s2 2p3 B 1s2 2s2 2p6 3s2 3p1 C 1s2 2s2 2p6 3s2 3p6 3d2 D 1s2 2s2 2p6 3s2 3p6 3d2 4s2 4 The pressure rating of a car tire is typically given to be 210 kPa. This is based on an internal air temperature of 25C. A driver decides to inflate the tires of his car to the rated pressure after driving at a high speed for some time, during which the internal air temperature has risen to 40C. What will be the drop in pressure in each tire when the temperature has returned to 25C? A 9.3 kPa B 10.1 kPa C 78.8 kPa D 200 kPa

3  VJC 2015 9647/01/PRELIM/15 [Turn over 5 LiAlH4 and NaBH 4 are used as a source of hydride ion s, H –, in many organic reactions. Both chemical formulae consist of a metal cation and an anion made up of a group of atoms. Solid LiA lH4 decomposes upon heating to 150 C while solid NaBH 4 melts at a temperature of 400 C. Which one of the following statements about LiAlH4 and NaBH4 is correct? A The H – from LiA lH4 is more available for donation due to the higher polarity of the Al–H bond compared to B–H bond in NaBH4. B The release of H – from LiA lH4 and NaBH 4 involves homo lytic fi ssion of a covalent bond. C The lower stability of LiAlH4 to heat can be explained by the higher charge density of the cation and anion present compared to NaBH4. D The presence of a distinct melting point for NaBH 4 means that the covalent bonds in the anions are weaker than the ionic bonds between the cations and anions. 6 The enthalpy change of the following half equation (Δ Hr o) is determined using Hess’s Law: ½ I2(s) + e– I–(aq) I(g) + e– I–(g) Which one of the following shows the correct signs for the different enthalpy changes? ΔH1 o ΔH2 o ΔH3 o A + + – B + – – C +