2023 ACSI Chemistry Y4 Prelim P3 Answers for students

ACS(Independent) 2023 Prelim Exam Y4 express 6092/3 1 ACS(Independent) Year 4 Express 2023 Prelim CHEMISTRY PAPER 3 1 a Heading + unit – 1m 2 readings +/- – 1m 2dp -1m Accuracy – 2m b Ave – 1m burette reading is 26.30cm3 c No of mol NaOH = 25/1000X0.1 = 0.0025 mol No of mol HXO3 = 0.0025 mol Concentration HXO3 = 0.0025/26.3X1000 = 0.095057 = 0.0951 mol/dm3 d 1 dm3 solution contains = 3.5 x 2 = 7.5 g HXO3 Mr of HXO3 = mass / mole = 7.5 / 0.095057 = 78.9 e Element Relative atomic mass Formula of its acid HXO3 Relative molecular mass of its acid Bromine (Br) 80 HBrO3 1 + 80 + 48 = 129 Chlorine (Cl) 35.5 HClO3 1 + 35.5 + 48 = 84.5 Nitrogen (N) 14 HNO3 1 + 14 + 48 = 63 Phosphorus (P) 31 HPO3 31 + 1 + 48 = 80 f X is phosphorus 2 a The filtrate is a pale blue solution b i Pungent, colourless gas turms most red litmus paper blue. Ammonia gas produced. ii White ppt formed with aqueous silver nitrate Ppt dissolves in aqueous ammonia to form a colourless solution c Filtrate is a blue solution i Light blue ppt, Ppt is soluble in excess to form a deep blue solution ii Cream ppt in a brown solution / filtrate is a brown/orange solution iii Blue ppt formed. Black ppt on warming d The metal is copper and the impurity is ammonium chloride e i Cu2+ (aq) + 2 OH- (aq)  Cu(OH)2 (s) ii Cu(OH)2 (s)  CuO (s) + H2O (l) f KI is a reducing agent, the iodide ion is oxidised to form brown iodine solution. Iodide ions lose electrons to form iodine. g All nitrate salts produced from nitric acid are soluble in water. The solutions are suitable for precipitation in the test reactions. However, a few salts of sulfate and chloride are not soluble and could not be used in precipitation.

ACS(Independent) 2023 Prelim Exam Y4 express 6092/3 2 i Independent variable: types of catalyst (X and MnO2) Dependent variable: volume of oxygen gas at 1 min Controlled variables: mass of catalyst; particle size of catalyst; concentration and volume of hydrogen peroxide; temperature 1 Set up the apparatus as shown in the diagram. 2 Measure 50cm3 of hydrogen peroxide using a measuring cylinder and transfer into the conical flask attached to a gas syringe. 3 Weigh 1.00g of X into a small test tube and secure it with a thread to the stopper of the conical flask as shown in the diagram. Release the thread and shake the conical flask for the hydrogen peroxide to react with the catalyst. 4 Measure the time taken to collect 50 cm3 of gas. 5 Repeat step 1 to 4 using 1.00g of manganese(IV) oxide Instead of X. 6 The results are tabulated. Method 1 Results and conclusions: Variable 1 Apparatus 1 catalyst Time taken to collect 50 cm3 of gas / s Procedure 1 X T1 MnO2 T2 T1 is longer than T2. MnO2 is a better catalyst than X. 3 c As the concentration of the dilute 00sulfuric acid increases, the time taken for the magnesium to dissolve decreases and hence the reaction is faster. d concentration of the acid = 0.0210/30X1000 = 0.7 mol/dm3 from the graph, time taken is 120s e Measure the mass of magnesium instead of using the length of 4 cm. Use burette or pipette to measure the fixed volume of acid. Dry the breaker after washing and rinsing. Gas syringe Conical flask 1g catalyst H2O2