Copy of PLMGSS 2024 4E Chem P1 Ans

Answers to Sec 4 Pure Chemistry prelim 2024 Qn Ans Explanation 1 C Strength of acids can be tested by measuring the temperature change during acid-metal reaction. Hence, thermometer would be required. pH is measured using the Universal Indicator or pH meter. Volume of sodium hydroxide needed for neutralisation and final volume of gas produced during acid-metal reaction will be similar, given that the concentrations of the unknown acid and hydrochloric acid are the same. 2 D Sodium hydroxide solution will neutralise the acidic carbon dioxide, concentration sulfuric acid will dry the gases, and copper will react with oxygen. Hence, nitrogen gas will remain. 3 B Rf values of the 3 spots are 0.3, 0.6 and 0.9 – distance travelled by ink divided by distance travelled by solvent front. Hence, only diethanolamine and oxybenzone are present. 4 A Mr of CH3Cl = 12 + 3 + 35.5 = 50.5 Mr of SO2 = 32 + 16x2 = 64 CH3Cl will travel the fastest due to its lower mass, on the hot day. 5 D S to T is where the sample will freeze and change from liquid to solid. Hence, this will result in a change in movement from moving around each other to vibrating about in fixed positions. 6 D P3− has 16 neutrons, 15 protons and 18 electrons. O2− has 8 neutrons, 8 protons and 10 electrons. Mg2+ has 12 neutrons, 12 protons and 10 electrons. Ne has 10 neutrons, 10 protons and 10 electrons. 7 C 32.1% x 64 + 56.4% x 66 + 11.5% x 67 = 65.5 54.6% x 64 + 6.6% x 66 + 38.8% x 67 = 65.3 56.3% x 64 + 31.3% x 66 + 12.6% x 67 = 65.1 53.5% x 64 + 25.5% x 66 + 21.2% x 67 = 65.3 8 B J has a charge of +2 – hence, it could be magnesium which is in Group 2. K has 5 valence electrons – hence, it should be in Group 15. L has 1 valence electron – hence, it should be hydrogen. It cannot be Group 1 metals as metals do not form covalent bonds. K and L shared 2 electrons – hence, they formed a covalent bond. 9 B Chloroacetone is a covalent compound, which will not be able to conduct electricity in any state, and has low melting and boiling points. There are 10 covalent bonds, hence there are 20 electrons involved in bonding. Chlorine shares one electron and has 6 valence electrons not involved in bonding. 10 D Based on the coordination number, each Y is surrounded by 6Z while each Z is surrounded by 4Y. This means that the ratio between Y and Z is 2:3. Hence, formula is Y2X3. 11 D Calcium hydroxide reacts with ammonium carbonate to give off ammonia gas, resulting in a loss in overall mass.

A and C show neutralisation reaction with no gas produced, while copper does not react with acid. 12 A Hydrogen reacts with oxygen to form water, which is neutral. Aluminium oxide is amphoteric, magnesium oxide is basic and sulfur dioxide is acidic. 13 A Lead(II) nitrate, calcium chloride and calcium nitrate are soluble. Lead(II) sulfate, lead(II) chloride and calcium sulfate are insoluble. 14 D Potassium is a very reactive metal, hence will react explosively with acid. Reaction is unsafe to proceed. 15 D Test for ammonium ions with reaction 1 and nitrate ions with reaction 3 could produce ammonia gas, which will turn moist red litmus paper blue. Hydrochloric acid is used to test for carbonate ions, which produces carbon dioxide gas. 16 A Displacement by chloride produces brown solution – iodine is produced. Precipitation of yellow precipitate – silver iodide is produced. No ppt with aqueous ammonia and sodium hydroxide – sodium, potassium or ammonium ions possible Hence, P is KI. 17 C R is copper(II) sulfate, which can be produced by reacting copper(II) carbonate with sulfuric acid. Copper is a transition metal with variable oxidation states. Q is copper(II) oxide which reacts with acids to give salt and water. Copper(II) ions give a blue ppt insoluble in excess sodium hydroxide. 18 B Number of moles of gases = 1.8 / 24 = 0.075 Number of moles of salt = 0.075/3 x 2 = 0.05 Mr of salt = 8.5 / 0.05 = 170 Ar of salt = 170 – 14 – (16x3) = 108 19 B Number of moles of ammonia = 17 000 000 / 17 = 1 000 000 Number of moles of ammonium nitrate = 1 000 000 / 2 = 500 000 Maximum mass of ammonium nitrate = 500 000 x (2x14 + 4 + 3x16) = 40 000 000 g = 40 tonnes 20 C 2NaOH + MgSO4 → Mg(OH)2 + Na2SO4 Number of moles of magnesium sulfate = 1.20 / (24 + 32 + 4x16) = 0.01 Mass of magnesium hydroxide = 0.01 x (24 + 2x16 + 2) = 0.58g Percentage yield = 0.32/0.58 x 100 = 55.2% 21 C Combustion and condensation are exothermic. Boiling/vapourisation is endothermic. 22 B P has lower energy level, hence R gives out energy to form P in the backward reaction – exothermic. Step 1 is endothermic but step 2 is exothermic. While step 1 has higher activation energy than step 2, it is because the amount of energy needed to break bonds is

higher – not because more bonds are broken. Step 1 involves breaking stronger bonds. 23 D Ammonia is formed via a reversible reaction, hence it will decompose when heated to a high temperature. Ammonia is alkaline, and will be formed when ammonium salts are heated with bases. Ammonia is made from hydrogen – obtained from cracking of petroleum, and nitrogen – obtained from fractional distillation of air. 24 C Experiment 1 is shown by curve Z – fastest reaction due to the use of powdered calcium carbonate, and greatest change in mass as HCl is used in excess. Experiment 2 is shown by curve Y – slower reaction due to the use of calcium carbonate in lumps. Experiment 3 is shown by curve X – slower reaction due to the use of calcium carbonate in lumps, and smaller change in mass due to insufficient HCl used. 25 C Reaction 1: oxidation state of sulfur remains the same (+6). Reaction 2: oxidation state of sulfur changes from +6 in H2SO4 to +4 in SO2 – change by 2. Reaction 3: oxidation state of sulfur changes from +6 in H2SO4 to -2 in H2S – change by 8. Comments: badly done. Many chose B. 26 A KMnO4 has been reduced as the oxidation state of Mn has decreased from +7 to +2, and acts as an oxidising agent. Ethanol is oxidised and acts as a reducing agent. 27 A Experiment 1: hydrogen is discharged at cathode and chlorine gas is discharged at anode. Experiment 2: hydrogen is discharged at cathode and chlorine gas is discharged at anode. Experiment 3: lithium is discharged at cathode and iodine gas is discharged at anode. 28 D Argon is inert and is used to prevent the oxidation of strontium. Molten strontium bromide is used as electrolyte as its ions act as mobile charge carriers to conduct electricity. 29 C Cu → Cu2+ + 2e Number of moles of copper = 4 / 64 = 0.0625 Number of moles of electrons = 0.0625 x 2 = 0.125 Cr → Crn+ + ne Number of moles of chromium = 2.17 / 52 = 0.0417 Number of moles of electrons = 0.0.417 x n = 0.0417n Same amount of charge for Cu and Cr: 0.0417n = 0.125 Hence, n = +3.0 30 B Cations are discharged at the cathode, hence the bracelet must be at the cathode. Silver anode will discharged to give silver ions, and the electrolyte must contain silver ions. 31 D Carbon dioxide is not a pollutant. Sulfur dioxide is from volcanic activity, methane from cattle farming, nitrogen dioxide from lightning flashes and carbon monoxide from incomplete combustion of petrol.

32 C Sulfur dioxide, sulfur trioxide, nitrogen dioxide and carbon dioxide are acidic gases that can be removed by calcium carbonate. 33 C Melting point increases then decreases across the period. Number of protons increases. Ability to conduct electricity increases then decreases. The number of electrons involved in bonding increases then decreases. 34 B They are soft and melting point decreases down the group. They are reducing agents and their reactivity increases down the group. 35 C 2 is the most reactive as it reacts with acid and its oxide cannot be reduced by heating with carbon. 3 is the least reactive as it cannot react with