MI 2021 PU1 H2 EOY P1 answers

2021 End-of-Year Examination Pre-University 1 H2 CHEMISTRY 9729/01 Paper 1 Multiple Choice & Structured Questions 15 Oct 2021 2 hours Section A – Multiple Choice 1 2 3 4 5 B B C D D 6 7 8 9 10 A C B D B 11 12 13 14 15 A C A C C 1 1 Correct. Empirical formula is the lowest whole number ratio of the number of atoms of each element. 2 Correct. Molecular formula is the number of atoms of each element present in one molecule. 3 Not correct. The relative molecular mass of C6H12 is 72.0 + 12.0 = 84.0, not 48.0. Relative molecular masses are calculated by summing the relative atomic masses of each element found in the Periodic Table, multiplied by the number of atoms.

2 2 CxHy + (𝑥 + 𝑦 4) O2 ⟶ 𝑥 CO2 + 𝑦 2 H2O initial volume / cm3 10 90 0 change in volume / cm3 -10 -50 +30 final volume / cm3 0 40 30 volume of oxygen gas unreacted = 40 cm3 volume of carbon dioxide gas formed = 70 – 40 = 30 cm3 volume of oxygen reacted = 90 – 40 = 50 cm3 𝜂𝐶𝑥𝐻𝑦 : 𝜂𝑂2 : 𝜂𝐶𝑂2 = 10 : 50 : 30 = 1 : 5 : 3 = 1 : 𝑥 + 𝑦 4 : 𝑥 hence 𝑥 = 3 𝑥 + 𝑦 4 = 5 𝑦 = 8 Molecular formula of hydrocarbon is C3H8. 3 A Not correct. BeCl2 is a linear molecule. Be only has 2 valence electrons, and forms two bond pairs with Cl. Be has no lone pairs of electrons. Hence the two electron domains around Be form a 180° from each other. B Not correct. BeCl2 is a molecule. Be 2+ ion has a high charge density, strongly polarising Cl- ion electron clouds towards itself, forming Be–Cl covalent bonds. C Correct. Be has an electronic configuration of 1s 2 2s2. To form 2 bonds, its valence orbitals (2s and 2p) undergo sp hybridisation, forming 2 sp orbitals which form covalent bonds with C l atoms. Be hence has 2 energetically -accessible vacant unhybridised 2p orbitals which can accept lone pairs of electrons from chlorine atoms, forming a polymer. D Not correct. The 1s orbital of Be is fully-filled and cannot accept additional electrons.

3 [Turn over 4 2Zn(NO3)2(s) ⟶ 2ZnO(s) + 4NO2(g) + O2(g) +5 -2 +4 0 Zn(NO3)2 was oxidised to form O2. The oxidation state of O increased from -2 (in Zn(NO3)2) to 0 (in O2). Zn(NO3)2 was also reduced to form NO2. The oxidation state of N decreased from +5 (in Zn(NO3)2) to +4 (in NO2). 5 Ground-state electronic configuration of Cl: 1s2 2s2 2p6 3s2 3p5 Excited-state electronic configuration of Cl: 1s2 2s2 2p6 3s1 3p6 One electron must have transferred from 3s (lower energy) to 3p (higher energy). 6 element electronic configuration number of half- filled orbitals A C 2 B F 1 C Mg 0 D Al 1 7 The first bond between two atoms is a sigma (σ) bond. Any bond formed in excess of the first one is a pi (π) bond. C C C N H H H 8 mel