2025 DHS H2 Chem Prelim Paper 1_Worked Solutions

2025 DHS Preliminary Examination H2 Chemistry 9729 Paper 1 Suggested Solutions © DHS Chemistry Unit Page 1 of 6 Answer Key 1 2 3 4 5 6 7 8 9 10 D B A C D A B D A C 11 12 13 14 15 16 17 18 19 20 D C B A C B B D D A 21 22 23 24 25 26 27 28 29 30 D C D C A C D B A C 1 D  A Since both particles have a positive angle of deflection, they are deflected in the same direction towards the same plate. Similar to 4He2+, X must be positively charged.  B The beam of positively charged particles X travels in a curved path towards the negatively charged plate.  C Charge to mass ratio of 12C+ = + 1 12 Charge to mass ratio of 4He2+ = + 2 4 = + 1 2 Since angle of deflection is proportional to charge to mass ratio, the angle of deflection of 12C+ is expected to be 1 6(1) = +0.17  D None of the above options are correct. 2 B 1. Determine the number of neutrons in Sr (find proton number of Sr using Data Booklet.) ⇒ 84 – 38 = 46 2. Find number of electrons present in Sr 2+ (using proton number of Sr given in Data Booklet.) ⇒ 38 – 2 = 36 ⇒ W has 36 electrons 3. Find number of protons present in W W: 36 protons 4. Calculate the nucleon number of W W: 36 + 46 = 82 The outer electronic configuration of W is 4s24p6 and hence a total of four orbitals (one 4s and three 4p) are occupied in the valence shell. 3 A (1, 2 and 3 only) The molecules have the following linear structures: O=C=O, S=C=S and O=C=S  1 C=O and C=S are polar covalent bonds due to the difference in electronegativity between the atoms.  2 Each C=O and C=S bond contains one sigma and one pi bond so each molecule contains two sigma and two pi bonds.  3 The dipole moments of the polar bonds are equal and opposite in O=C=O and S=C=S molecules so they cancel out and the molecules are non -polar. There exists only instantaneous dipole –induced dipole ( id–id) interactions between these non –polar molecules. As O is more electronegative than S, C=O bond is more polar than C=S bond and there is a net dipole moment in O=C=S. Hence, COS molecule is polar and it has both id –id and permanent dipole –permanent dipole interactions between molecules.  4 O=C=S has a smaller, less polarisable electron cloud than S=C=S. Hence, O=C=S has weaker id-id interactions between molecules than S=C=S. 5 D 2H2S + 3O2 → 2SO2 + 2H2O CS2 + 3O2 → CO2 + 2SO2 Hence, SO2 : CO2 will be 4 : 1.  Options A & C are incorrect. In the 60 cm 3 mixture, there is 40 cm 3 of H 2S and 20 cm3 of CS2. 40 cm3 of H2S will form 40 cm3 of SO2. 20 cm3 of CS2 will form 20 cm3 of CO2 and 40 cm3 of SO2. Hence, total volume of acidic gases, CO2 and SO2, = 40 + 20 + 40 = 100 cm3 These acidic gases will react with NaOH(aq) and cause a reduction in gas volume. 6 A  A pV = nRT = mRT/M  density = m/V = pM/RT Since density = pM/RT and M, T and R are constants, density is di