2022 HCI Prelim P1 Worked Solutions

2022 HCI C2 H2 Chemistry Prelims / Paper 4 HWA CHONG INSTITUTION 2022 C2 H2 CHEMISTRY PRELIMINARY EXAM SUGGESTED SOLUTIONS (PAPER 1) Paper 1 1 2 3 4 5 6 7 8 9 10 C B B D D D C A B A 11 12 13 14 15 16 17 18 19 20 B A A B B C C D B A 21 22 23 24 25 26 27 28 29 30 A C A D C A D D C D 1 C Option D can be eliminated immediately as He contains 2 protons. Option No. of protons No. of neutrons No. of electrons A 1 0 2 B 1 1 3 C 1 2 3 2 B The large drop in the graph suggests that A + ion (which has already lost its first electron) has electronic configuration of ns 1. Hence A originally has electronic configuration of ns 2 and is a Group 2 element. Since the graph is showing consecutive elements in the Period Table, B is a Group 13 element. 3 B Option Explanation A Methanol forms favourable hydrogen bonding between its –OH group and that in water. B The hydrogen atom is not bonded to F, O or N atom, hence there is no intermolecular hydrogen bonding for HBr and HCl. C For 2-aminophenol, the –NH2 and the –OH groups are in close proximity which allows for intramolecular hydrogen bonding. Hence there is less extensive intermolecular hydrogen bonding for 2- aminophenol and less energy is required to overcome the intermolecular forces, leading to a lower boiling point. D When ethanoic acid, CH3CO2H (Mr = 60), is dissolved in a non-polar solvent (hexane), the molecules form hydrogen-bonded dimers.

4 D Option Explanation 1 about 107 109.5 2 about 104.5 90 3 90 180 5 D Recall that real gases would deviate from ideal behaviour at high pressures and low temperatures. Option Explanation A HCl is a polar molecule, which will deviate less at higher temperature since the molecules have more kinetic energy to overcome the intermolecular permanent dipole -permanent dipole (pd -pd) interactions. B CH4 is a non-polar molecule with only weak intermolecular dispersion forces. CH 4 will also deviate less at higher temperature since the molecules have more kinetic energy to overcome the intermolecular forces. C O2 is a non-polar molecule with only weak intermolecular dispersion forces, hence should deviate less from ideal behaviour than HCl. D HF is a polar molecule with stronger hydrogen bonding interactions than the permanent dipole-permanent dipole interactions of HCl. Hence HF deviates more from ideal behaviour than HCl at the same temperature of 300 K. 6 D An acid -base indicator is a weak aqueous acid whose acid form, HIn, is a different colour from its ionised form (or conjugate base form), In –. As such, it shows different colours in solutions of different pH. HIn(aq) ⇌ H+(aq) + In–(aq) red yellow

2022 HCI C2 H2 Chemistry Prelims / Paper 4 7 C When the metal nitrate decomposes, metal oxide, nitrogen dioxide and oxygen are formed. M(NO3)2 d MO + 2NO2 + ½O2 The metal oxide then reacts with the added acid to fo