SAJC Prelim P4 QP

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 you. If you are unsure if the blue colour has discharged or not, you may also add a few more drops of thymolphthalein to the solution. Do not discard this mixture! (The volume of FA 2 required to reach the first end–point need not be consistent but it should not exceed 15 cm3.) 4. Add 3 drops of screened methyl orange to this mixture and continue the titration until the second end–point is reached. Record the final burette reading in the same table provided on page 3. 5. Perform sufficient titrations to obtain consistent results for the second end–point, which is the total volume of FA 2 required.

3 Results 1 2 3 Final burette reading (with screened methyl orange) / cm3 Final burette reading (with thymolphthalein) / cm3 Initial burette reading / cm3 Total volume of FA 2 used to reach the second end–point / cm3 Place a tick for the consistent results [4] (b) x is the average volume of FA 2 required to reach the first end point. y is the average further volume of FA 2 required to reach the second end–point. From your 2 consistent titration results, calculate x and y. x = ……………………. y = ……………………. [2] (c) (i) Using x, calculate the amount, in moles, of Na 2CO3 in 25.0 cm3 of FA 1. Number of moles of Na 2CO3 = ……………………… [1]

4 (c) (ii) Using y, calculate the total amount, in moles, of NaHCO3 in the reaction mixture. Total number of moles of NaHCO3 = ……………………… [1] (d) From your answers in (c), calculate the amount, in moles, of NaHCO3 in 25.0 cm3 of FA 1. Number of moles of NaHCO 3 = ……………………… [1] (e) Calculate the percentage by mass of sodium carbonate in FA 1, which is a mixture of NaHCO 3 and Na2CO3. [Ar: Na, 23.0; H, 1.0; C, 12.0; O, 16.0] [2]

5 (f) A student decides to carry out the same experiment in (a) by using a pH meter to determine the percentage by mass of Na2CO3 in the mixture. Given that pKb of CO32 ̶ = 3.68 and pKb of HCO3 – = 7.62, sketch a graph to show how the pH of the mixture changes as HCl is added. You may use your titre values in any set of experiments obtained in (a) for the volume of HCl at the two equivalence points. [2] Planning (g) Unlike Group 2 carbonates, solid sodium carbonate, Na 2CO3, does not decompose on heating with a Bunsen burner. However, for solid sodium hydrogencarbonate, NaHCO 3, it will form sodium carbonate, steam and carbon dioxide when heated. (i) Write an equation, with state symbols, to represent the thermal decomposition of sodium hydrogen carbonate. ………………………………………………………………………………………………….. [1]

6 (g) (ii) Given the above information, you are to dev ise a plan to determine the percentage by mass of NaHCO3 in a solid mixture of NaHCO3 and Na2CO3. You may assume you are provided with • a boiling tube • a pair of tongs • 10 g of NaHCO 3 and Na2CO3 solid mixture • a heat resistant mat • equipment normally found in a school or college laboratory In your plan, you should include details of: • procedure you would follow, including t he apparatus you would use and the safety precautions to take, • measurements you would take, • a brief outline of how the results can be used to determine the percentage by mass of NaHCO3 in the mixture. You may make assumptions for the masses involved and use appropriate molar mass values to solve this. [Ar: Na, 23.0; H, 1.0; C, 12.0; O, 16.0] ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ………………………………………………………………………………………………….. [6] [Total: 20]

7 2 Determination of n in the complex ion formula [Cu(NH 3)n]2+ Many transition metal ions possess the ability to form complexes with ligands, which can either be organic or inorganic molecules/ions. In this experiment, you will determine the value of n in the formula [Cu(NH3)n]2+(aq), which is the complex ion formed when aqueous NH3 is added to Cu2+(aq). Different volumes of CuSO 4 solution and aqueous ammonia are mixed and the corresponding temperature changes are measured.