NJC Prelim P4_Question_Paper

© NJC 2017 9729/04/Prelim17 [Turn over NATIONAL JUNIOR COLLEGE SH2 Year-End Practical Examination Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 4 Practical Candidates answer on the Question paper Additional Materials: As listed in the Confidential Instructions 9729/04 17 August 2017 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your identification number and name. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 19 and 20. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 20 printed pages including this cover page. Shift Laboratory For Examiner’s use 1 / 12 2 / 16 3 / 10 4 / 17 Total / 55

2 © NJC 2017 9729/04/Prelim17 Answer all the questions in the spaces provided. 1 Determination of x in the formula of hydrated magnesium carbonate, MgCO3.xH2O. Magnesium carbonate is a white, powdery compound that occurs naturally as anhydrous and several hydrated forms. Due to its non-toxicity, it is widely marketed in common products such as cosmetics and toothpaste. A sample of a hydrated magnesium carbonate mineral is analysed to find out the number of moles of water per mole of magnesium carbonate. You are given the following: FA 1 is MgCO3.xH2O solid. FA 2 is 2.00 mol dm–3 nitric acid, HNO3. FA 4 is 9.10 g dm3 sodium hydroxide, NaOH. Indicator: methyl orange In this question, you will perform a back titration. The data from this titration will be used to determine the Mr of MgCO3.xH2O and hence the value of x. (a) In this titration, you will react MgCO3.xH2O with an accurately measured amount of excess FA 2. The reaction mixture was then used to prepare bulk solution FA 3. The amount of unreacted FA 2 in FA 3 is then determined via titration with FA 4. The end -point is reached when the colour of the solution changes from pink to yellow. (i) Preparation of FA 3 by reacting FA 1 with excess FA 2 Record all the mass measurements in the space provided on page 3. 1. Weigh out accurately about 0.20 g of FA 1 in a clean and dry weighing bottle. 2. Transfer FA 1 to a clean and dry small beaker. Measure and record the mass of weighing bottle and residual solid. Calculate the actual mass of FA 1 transferred. 3. Pipette 25.0 cm3 of