GESS 4Exp Pure Chem Prelim 2024_Answers

1 GESS Sec 4 Chemistry (6092) Preliminary Exams 2024 Suggested Solutions PAPER 1 Question Number Key Question Number Key 1 A 21 B 2 B 22 A 3 B 23 D 4 C 24 D 5 B 25 B 6 B 26 A 7 C 27 B 8 D 28 A 9 A 29 A 10 D 30 B 11 A 31 D 12 A 32 C 13 C 33 D 14 A 34 B 15 A 35 B 16 C 36 C 17 D 37 D 18 A 38 D 19 A 39 B 20 C 40 D

2 Question Answer Explanation 1 A Option A is correct as the burette can measure variable volumes of a liquid in a titration. Option B is incorrect because the 0.24 dm3 is equivalent to 240 cm3 which exceeds the capacity of the gas syringe. Option D is incorrect as a pipette can only measure a fixed volume of a liquid depending on its size, in this case 25.0 cm3. 2 B Q has a lower Rf value than R as it travelled a shorter distance from the start line. 3 B The mixture is heated using a water bath, hence any substance with a boiling point above 100 ºC (i.e. C & D) cannot be distilled. Substance A is a gas at room temperature. 4 C The transition from (s) → (l) → (g) is endothermic as energy is absorbed to overcome forces of attraction between the particles. 5 B A mixture of solid and liquid exists at the melting point/freezing point, which occurs at 50 ºC. 6 B Since the ion has a charge of 3+, it means that Z would have lost 3 electrons, and is therefore in Group 13. Since the ion has 2 electron shells, Z must have had three electron shells in total and is hence in Period 3. 7 C Sea water is a mixture; sodium chloride is a compound and chlorine is an element. 8 D In graphite, each carbon atom is only bonded to three other carbon atoms, leaving one electron per carbon atom not involved in bonding. The electrons which are not involved in bonding are delocalised, acting as mobile charged carriers. Metals such as aluminium have a sea of delocalised electrons which act as mobile charged carriers. Ionic compounds conduct electricity due to the presence of mobile ions. 9 A The electronic structure of oxygen is 2,6. It needs 2 more electrons to have a fully-filled valence shell. Each oxygen atom forms 2 covalent bonds. The electronic structure of chlorine is 2,8,7. It needs 1 more electron to have a fully-filled valence shell. Each chlorine atom forms 1 covalent bond. This results in a structure with the formula Cl2O. 10 D Both silicon and carbon are in Group 14, hence SiO2 and diamond both have a giant covalent structure where the atoms around Si and C are in a tetrahedral arrangement, i.e. each Si and C atom forms 4 covalent bonds. For giant covalent structures, there are no intermolecular forces of attraction as all the atoms are joined via covalent bonds. Melting requires breaking these covalent bonds. 11 A nO2 = 500/24000 = 0.020833 mol 1 mol contains 6.02 × 1023 molecules. Hence, number of molecules = 0.020833 × 6.02 × 1023 = 1.254 × 1022

3 12 A The sum of the relative molecular masses of Mg atoms in a molecule of chlorophyll-a is !.#$%&&×893 = 24.02. Since the Ar of a Mg atom is 24,