2017 RVHS H2 Chemistry P4 Soln

River Valley High School Pg 1 of 12 Year 6 H2 Chemistry 9729 Practical Examination 2017 RIVER VALLEY HIGH SCHOOL YEAR 6 PRACTICAL EXAMINATION H2 CHEMISTRY 9729 23RD AUG 2017 2.5 HOURS NAME CLASS 6 ( ) INDEX NO. INSTRUCTIONS TO CANDIDATES DO NOT OPEN THIS BOOKLET UNTIL YOU ARE TOLD TO DO SO. Read these notes carefully. Write your name, class and index number in the spaces at the top of this page. Give details of the practical shift and la boratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graph. Do not use staples, paper clips, highlighters, 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 Note s are printed on pages 14 and 15. _________________________________________________________________________________ Answer all the questions in the spaces provided. Shift Laboratory For Examiner’s Use 1 2 Total For Examiner’s Use 1 / 21 2 / 15 3 / 09 4 / 10 Total / 55

River Valley High School Pg 2 of 12 Year 6 H2 Chemistry 9729 Practical Examination 2017 1 Determine the the percentage by mass of sodium ethanedioate in a mixture of sodium ethanedioate and ethanedioic acid. This experiment involves two steps. In step one, you will carry out a titration to find the amount of acid, H2C2O4, present in FB 3. In step two, you will carry out a second titration to find the total amount of ethanedioate ion, C2O42–, present in FB 3. Finally, you will use the values found in the two steps to calculate the percentage by mass of sodium ethanedioate in FB 3. FB 1 is 0.100 mol dm–3 sodium hydroxide, NaOH. FB 2 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FB 3 is a mixture of aqueous sodium ethanedioate, Na2C2O4, and ethanedioic acid, H2C2O4. FB 4 is approximately 2 mol dm–3 sulfuric acid. thymolthalein indicator Read through the whole method before starting any practical work. (a) Method Step 1 1. Fill the burette labelled FB 1 with FB 1. 2. Pipette 25.0 cm 3 of FB 3 into a conical flask. 3. Add 1 dropper full of thymolphthalein. 4. Titrate FB 3 in the conical flask with FB 1 until a pale blue colour is seen. 5. Carry out as many accurate titrations as you think necessary to obtain consistent results. 6. Record in a suitable form below all of your burette readings and the volume of FB 1 added in each accurate titration. Step 2 1. Pipette 25.0 cm 3 of FB 3 into a conical flask. 2. Using a measuring cylinder, add about 25 cm 3 of 2 mol dm–3 sulfuric acid, FB 4, to the flask. 3. Place the conical flask on a hotplate and heat to about 65°C. 4. Fill the burette labelled FB 2 with FB 2. 5. Use an appropriate method to carefully transfer the hot conical flask onto a white tile under the burette. 6. Titrate the mixture in the conical flask with FB 2 until a permanent pale pink colour is seen. If a permanent brown colour is seen, stop the titration and begin Step 2 again. 7. Carry out as many accurate titrations as you think necessary to obtain consistent results. 8. Record in a suitable form below all of your burette readings and the volume of FB 2 added in each accurate titration. [3] (b) (i) From your titration results in Step 1 , obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value. Shift 1 Shift 2 Shift 3 15.20 cm3 15.20 cm3 15.20 cm3

River Valley High School Pg 3 of 12 Year 6 H2 Chemistry 9729 Practical Examination 2017 25.0 cm3 of FB 3 required ………….. cm 3 of FB 1 [2] (b) (ii) Write an equation for the reaction between sodium hydroxide and ethanedioic acid to give sodium ethanedioate and water. H2C2O4 + 2NaOH ¹ Na2C2O4 + 2H2O …………………………………………………… ……………………………………………...... [1] (b) (iii) Use your answer from (b)(i) to calculate amount of sodium hydroxide, FB 1, required to react with 25.0 cm3 of FB 3 in Step 1. (b)(i) × 0.10}/1000 Amount of NaOH = ………………………. [1] (b) (iv) Use your answer to (b)(iii) to determine the amount of ethanedioic acid in 25.0 cm3 of FB 3. (iii)/2 Amount of C2O4H2 in 25.0 cm3 of FB 3 = .............................. [1] (c) (i) From your titration results in Step 2 , obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value. Shift 1 Shift 2 Shift 3 22.60 cm3 22.75 cm3 22.60 cm3 25.0 cm3 of FB 3 required ………….. cm 3 of FB 2. [3] (c) (ii) Use your answer from (c)(i) to calculate amount of potassium manganate(VII), FB 2, required to react with 25.0 cm3 of FB 3 in Step 2. (c)(i) × 0.02/1000 Amount of KMnO 4 = .............................. [1] (c) (iii) The equation for the reaction between acidified manganate(VII) ions and ethanedioate ions is shown below. 2MnO4– (aq) + 5C2O42– (aq) + 16H+ (aq) ¹ 2Mn2+ (aq) + 10CO2(g) + 8H2O(l) Calculate the total amount of ethanedioate ions in 25.0 cm3 of FB 3. (c)(ii) × 5/2 Total amount of C2O42– in 25.0 cm3 of FB 3 = ……………… [1] (c) (iv) Use your answers to (b)(iv) and (c)(iii) to calculate the amount of ethanedioate ions which came from the sodium ethanedioate dissolved in 25.0 cm3 of FB 3. (c)(iii) ⚶ (b)(iv)

River Valley High School Pg 4 of 12 Year 6 H2 Chemistry 9729 Practical Examination 2017 Amount of C2O42– from C2O4Na2 in 25.0 cm3 of FB 3 = ………………. [1] (d) (i) Use your answer to (b)(iv) to calculate the mass of ethanedioic acid, H 2C2O4, in 25.0 cm3 of FB 3. [Ar: H, 1.0; C, 12.0; O, 16.0] (If you were unable to answer (b)(iv), you may assume that the amount of ethanedioic acid is 6.51 × 10–4 mol.) (b)(iv) × 90.0 Mass of ethanedioic acid = ........................ [1] (d) (ii) Use your answer to (c)(iv) to calculate the mass of sodium ethanedioate, Na2C2O4 in 25.0 cm3 of FB 3. [Ar: C, 12.0; O, 16.0; Na, 23.0] (If you were unable to answer (c)(iv), you may assume that the amount of sodium ethanedioate is 4.13 × 10–4 mol.) (c)(iv) × 134.0 Mass of sodium ethanedioate = ........................ [1] (d) (iii) Calculate the percentage by mass of sodium ethanedioate present in FB 3. {mass Na2C2O4 in (ii)/total mass} × 100 [total mass = (d)(i) + (d)(ii)] Percentage by mass of sodium ethanedioate present is ...................... [1] (e) (i) A student suggested that using a burette to measure the 25.0 cm 3 of acid would give a more accurate result than using a pipette. The percentage error of a 25.0 cm 3 pipette is 0.24 %. Is the student correct? Explain your answer. [2] Student is incorrect use of burette: {0.10/25} × 100 = 0.40% compared to 0.24% or apparatus error of pipette is ±0.06 compared with apparatus error of burette is ±0.10 (e) (ii) A student decided to use a 25.0 cm3 pipette instead of a measuring cylinder to measure the volume of FB 4 in Step 2. State and explain whether this alteration will improve the accuracy of the calculation of the percentage by mass of sodium ethanedioate in the mixture. [2] No improvement as acid in excess

River Valley High School Pg 5 of 12 Year 6 H2 Chemistry 9729 Practical Examination 2017 2 Investigate how the rate of the following reaction varies with the concentration of sodium thiosulfate, Na2S2O3. Na2S2O3 (aq) + H2SO4 (aq) → S (s) + Na2SO4 (aq) + SO2 (g) + H2O (l) The rate can be found by measuring how long it takes for the solid sulfur formed to obscure the printing on the insert provided. Care should be taken to avoid inhalation of SO 2 (g) that is given off during this reaction. FC 5 is 1.0 mol