SAJC Prelim P4_Answers

1 Name: Shift: Class: Laboratory: ST ANDREW’S JUNIOR COLLEGE PRELIMINARY EXAMINATIONS PAPER 4 PRACTICAL Chemistry Higher 2 28 August 2018 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST. Write your name and class on all the work you hand in. Give details of the practical shift and laboratory in the boxes provided above. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all the 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 16 and 17. At the end of the examination, fasten all your work securely together. The number of marks is given in the brackets [ ] at the end of each question or part question. For Examiner’s Use 1 2 3 Total This document consists of 17 printed pages including this page.

2 Answer all the questions in the spaces provided. 1 Determination of percentage by mass of sodium carbonate in a mixture of sodium carbonate and sodium hydrogen carbonate The complete reaction between sodium carbonate and hydrochloric acid can be represented as shown: Na 2CO3 + 2HCl → 2NaCl + CO2 + H2O However, when sodium carbonate is titrated against hydrochloric acid, it reacts with hydrochloric acid in two separate stages. Na2CO3 + HCl → NaCl + NaHCO3 ---------------- reaction 1 NaHCO3 + HCl → NaCl + H2O + CO2 ---------------- reaction 2 When a mixture of sodium carbonate and sodium hydr ogen carbonate is titrated against hydrochloric acid using thymolphthalein indicator, the blue colour of the indicator is discharged when sodium carbonate is partially neutralised as represented by reaction 1. Screened methyl orange is then added and a further quantity of hydrochloric acid is necessary to bring about a colour change when reaction 2 is complete. This further amount of acid reacts with both the sodium hydrogen carbonate formed in reaction 1 as well as the sodium hydrogen carbonate present initially in the mixture. (a) In this experiment, you will determine the amount of Na 2CO3 and NaHCO 3 in FA 1 by following the procedures below. FA 1 is a solution containing sodium carbonate, Na 2CO3 and sodium hydrogen carbonate, NaHCO3. FA 2 is an aqueous solution containing 0.50 mol dm–3 of hydrochloric acid, HCl. 1. Pipette 25.0 cm3 of FA 1 into a conical flask. 2. Add 5 drops of thymolphthalein indicator and titrate FA 1 against FA 2 in a burette. 3. When the blue colour is discharged, note and record the burette reading in the table provided on page 3. This end-point may not be easily seen. Therefore, you are advised to put a white tile under your conical flask to help yo