2020 HCI H2 Chemistry P1 Answers

2020 HCI C2 H2 Chemistry Prelim Exam / Paper 1 HWA CHONG INSTITUTION 2020 C2 H2 CHEMISTRY PRELIMINARY EXAMINATION SUGGESTED SOLUTIONS Paper 1 1 2 3 4 5 6 7 8 9 10 D B C B D C B C B D 11 12 13 14 15 16 17 18 19 20 A B A D A B D C A D 21 22 23 24 25 26 27 28 29 30 C C C A A D B A B C 1 D This question tests the writing of electronic configurations. The electronic configuration and the number of unpaired p electrons for each of the species are shown below. A Al2−: 1s2 2s2 2p6 3s2 3p3 (3 unpaired p electrons) O+: 1s2 2s2 2p3 (3 unpaired p electrons) B N: 1s2 2s2 2p3 (3 unpaired p electrons) Cl2+: 1s2 2s2 2p6 3s2 3p3 (3 unpaired p electrons) C C: 1s2 2s2 2p2 (2 unpaired p electrons) Cl+: 1s2 2s2 2p6 3s2 3p4 (2 unpaired p electrons) D B−: 1s2 2s2 2p2 (2 unpaired p electrons) S+: 1s2 2s2 2p6 3s2 3p3 (3 unpaired p electrons) 2 B A simple molecular solid should have a low melting point (due to weak intermolecular forces) and does not conduct electricity (due to lack of mobile charge carriers – ions or free electrons). It may be soluble or insoluble in water depending on the type of intermolecular forces that it can form with water. Since the unknown compound is a solid at room temperature, th e answer cannot be A due to the melting point. 3 C A A  bond involves head-on overlap of the orbitals of two bonded atoms, thus electron density is highest between the two atoms e.g. A is a correct statement.

1 B This is a correct statement. An example of such a compound would be MgCO3 where ionic bonds exist between Mg2+ and CO32− while covalent bonds exists within CO32−. C A triple bond consists of two  bonds and one  bond instead. C is incorrect. D This is a correct statement. This is also part of the reasoning how cis-trans isomerism arises in alkenes. 4 B This question tests your understanding of ideal gas and involves the use of the ideal gas equation PV = nRT. A PV = nRT PV = (mass/Mr)RT Since density = mass / V, P × Mr = density × R × T density = P × Mr/(RT) Thus, the density of an ideal gas at constant temperature is directly proportional to the pressure. B PV = nRT V = 1 mol × RT / P At the same T and P, 1 mol of any ideal gas has the same volume V. This statement is correct. C As seen from the ideal gas equation, V is directly proportional to temperature (in Kelvins), for a given mass (or mol) of the ideal gas at constant P. However, when temperature is increased from 25 C (298K) to 50 C (323K), temperature is not doubled. Thus V is not doubled. D Since the volume of container does not change, the partial pressure of the ideal gas should not change as well. We may also think in terms of: Adding that second gas increases total pressure. However, the mole fraction of the first gas decreases by the same factor, hence its partial pressure does not change. 5 D According to the question, all the carb