Question 114 marks
Question
Ethanedioic acid forms salts with Group 1 metals. In this experiment you will identify the Group 1 metal ion, Z+, present in an ethanedioate salt, (COO)2Z2. You will titrate a solution of the salt with acidified aqueous potassium manganate(VII). The equation for the reaction between manganate(VII) ions and ethanedioate ions in acidic solution is shown. 2MnO4–(aq) + 16H+(aq) + 5(COO)22–(aq) → 2Mn2+(aq) + 8H2O(l) + 10CO2(g) FA 1 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FA 2 is a solution containing 8.06 g dm–3 of an ethanedioate salt, (COO)2Z2. FA 3 is dilute sulfuric acid, H2SO4. (a) Method ●● Fill the burette with FA 1. ●● ipette 25.0 cm3 of FA 2 into a conical flask. P ●● Use the measuring cylinder to transfer 25 cm3 of FA 3 into the same conical flask. ●● Place the conical flask on the tripod and gauze and heat the conical flask until the temperature of the solution is approximately 70 °C. ●● Carefully remove the hot conical flask and place it on the white tile under the burette. ●● During titrations, add FA 1, slowly at first, until a permanent pale pink colour is formed. (The pink colour on initial addition may take several seconds to disappear.) If the reaction mixture turns brown, reheat it to approximately 70 °C. If the brown colour disappears, continue with the titration. If the brown colour remains, discard the contents of the flask and begin a new titration. ●● Perform a rough titration (the end‑point is a permanent pale pink colour) and record your burette readings in the space below. The rough titre is …… cm3. ●● Carry out as many accurate titrations as you think necessary to obtain consistent results. ●● Make sure any recorded results show the precision of your practical work. ●● Record all of your burette readings and the volume of FA 1 added in each accurate titration. I II III IV V VI VII [7] (b) From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 2 required …… cm3 of FA 1. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to the appropriate number of significant figures. [1] (ii) Calculate the number of moles of manganate(VII) ions in the volume of FA 1 calculated in (b). moles of MnO4– = …… mol [1] (iii) Use the equation on page 2 to calculate the number of moles of ethanedioate ions in 25.0 cm3 of FA 2. moles of (COO)22– = …… mol [1] (iv) Calculate the relative formula mass, Mr, of the ethanedioate salt, (COO)2Z2. Mr of ethanedioate salt = …… [1] (v) Calculate the relative atomic mass, Ar, of the Group 1 metal, Z, in the ethanedioate salt. Show your working. Ar of Z = …… Hence identify Z. Z is …… . [2] [Total: 14]
Mark scheme
1(a) I The following data is shown 1 • two burette readings for the rough titration • titre for rough titration • initial and final burette readings for two (or more) accurate titrations II Appropriate headings and units for accurate titration. 1 and volume FA 1 added recorded for each accurate titre. Headings should match readings. • initial / start and (burette) reading / volume • final / end and (burette) reading / volume • titre or volume / FA 1 and used / added • unit: / cm3 or (cm3) or in cm3 or cm3 for each entry III All accurate burette readings are to the nearest 0.05 cm3. 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 Award V, VI and VII if δ ⩽ 0.20 (cm3) 3 Award V and VI if 0.20 < δ ⩽ 0.50 Award V, only, if 0.50 < δ ⩽ 0.80 1(b) Calculation of mean 1 • Candidate must average two (or more) accurate titres where the total spread is ⩽ 0.20 cm3. • Working must be shown or ticks must be put next to the two (or more) accurate readings selected. • The mean should normally be quoted to 2 dp rounded to the nearest 0.01 cm3. (e.g.: 26.666 must be rounded to 26.67 cm3. 1(c)(i) All numerical answers in 1(c) are given to 3 or 4 sf 1 1(c)(ii) Correctly calculates 1 0.02 × (b) / 1000 1(c)(iii) Correctly uses 1 answer (ii) × 5 / 2 (or equivalent) 1(c)(iv) Correctly uses 1 8.06 / (40 × answer (iii)) 1(c)(v) Correct expression 1 (answer (iv) – 88) ÷ 2 1(c)(vi) Correct answer 1 K / potassium Allow ecf of group 1 metal ion: (Li < 15.0; 15.0 ⩽ Na <31.1; 31.1 ⩽ K ⩽ 62.3; 62.3 ⩽ Rb ⩽ 109.2; 109.2 ⩽ Cs)