2024 Prelims RI H2 Chem P1 (Ans)

© Raffles Institution 2024 9729/01/S/24 2024 Y6 H2 Chemistry Preliminary Exams Paper 1 – Suggested Solutions Question 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Answer A B B C C D B D C A B A D A A MCQ worked solutions Q1 (Ans: A) At r.t.p, number of moles of molecules in 1 cm3 of CH4 gas = 1 24000 mol number of molecules in 1 cm3 of CH4 gas = 6.02 × 1023 24000 Q2 (Ans: B) CxHy(g) + (x+ y 4 )O2 (g) → xCO2(g) + y 2H2O(l) Volume of CO2 produced = 100 − 40 = 60 cm3 Volume of unreacted O2 = 40 cm3 Volume of O2 reacted = 130 – 40 = 90 cm3 Hence, x = 60 20 = 3. (x+ y 4 ) = (3+ y 4 ) = 90 20 y = 6 Thus, formula of the hydrocarbon is C3H6. Q3 (Ans: B) Option A is incorrect as 18O2− and 19F− each has 10 electrons and 10 neutrons. Option B is correct as both 18O2− and 19F− have 10 neutrons each. Option C is incorrect as both ions have outer electronic configuration of 2s2 2p6. Option D is incorrect as nucleons refer to protons and neutrons. Hence 18O2− has 18 nucleons and 19F− has 19 nucleons. Q4 (Ans: C) After the beta decay, the mass number remains the same at 234 and the resulting species has 90 + 1 = 91 protons, which is protactinium (Pa). Q5 (Ans: C) Option A is incorrect as boiling involves overcoming intermolecular forces (hydrogen bonding) and does not involve the breaking of covalent bonds. Furthermore, the F−H bond is stronger than the O−H bond. Option B is incorrect as both H2O and HF are isoelectronic (same number of electrons). Option C is correct as, on average, each H2O can form 2 hydrogen bonds per molecule whereas each HF only forms 1 hydrogen bond per molecule. Option D is incorrect as the statement is not a major reason for the higher boiling point of H 2O since hydrogen bonding is the predominant intermolecular force of attraction. Question 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Answer D B C A D B D B C C D D B C A

© Raffles Institution 2024 9729/01/S/24 Q6 (Ans: D) Option A is incorrect as ice is less dense than water due to its open structure, preventing the molecules from getting too close to one another . This also causes the same mass of H 2O to occupy a larger volume. Option B is incorrect as covalent O−H bonds are stronger than hydrogen bonds. Option C is incorrect as each O atom forms 2 hydrogen bonds, hence 4 electrons (2 electron pairs) are involved. Option D is correct as each H2O molecule is tetrahedrally bonded to four H atoms via covalent and hydrogen bonds. Q7 (Ans: B) Option 1 is correct to adhere to the given molecular formula and non-cyclic structure. Option 2 is incorrect as isomers 2 and 3 are optically active as they lack an internal plane of symmetry. However, both do not contain any chiral C atom. Option 3 is correct as there is a total of five non-cyclic isomers, including stereoisomers. Q8 (Ans: D) pV = nRT Option A is incorrect as p ∝ T, in K , at constant V . Since units of T on x-ax