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, henc