SAJC Prelim P4 Qns

1    Name: Shift: Class: Laboratory: ST ANDREW’S JUNIOR COLLEGE PRELIMINARY PRACTICAL EXAMINATION Chemistry Higher 2 29 August 2017 2 hours 30 minutes Additional Materials: Qualitative Analysis notes 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. 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. Planning Practical 14 41 This paper consists of 18 printed pages including this page and quantitative analysis notes.

2      1 Determination of the Mr of a hydrated ethandioate salt Calcium ethanedioate is the major component of the most common type of human kidney stones. It is one of a series of salts formed from ethanedioic acid, H2C2O4. Another of these salts can be represented by the formula X2C2O4.H2O, where X is a Group 1 metal. Solution Q contains 64.5 g dm-3 of X2C2O4.H2O in deionised water. You are not provided with Q. FA 1 is a diluted solution of Q, in which 35.70 cm 3 of Q was made up to 250 cm 3 with deionised water in a graduated flask. FA 2 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FA 3 is 1.00 mol dm–3 sulfuric acid, H2SO4. In this question, you will perform a titration. The data from this titration will be used to determine:  the concentration of C 2O42- in Q,  the Mr of X2C2O4.H2O, and hence the identity of X. (a) Titration of FA 1 against FA 2 In this titration, FA 2 is run from the burette into the conical flask containing FA 1 and FA 3. Initially, the colour of the FA 2 will take some time to disappear. After some FA 2 has been added, sufficient Mn 2+(aq) ions will be present to allow the reaction to occur faster. The end-point is reached when a permanent pale pink colour is obtained. (i) 1. Fill the burette with FA 2. 2. Using a pipette, transfer 25.0 cm3 of FA 1 into the conical flask. 3. Using an appropriate measuring cylinder, transfer 50.0 cm3 of FA 3 to the same conical flask. 4. Heat this solution to about 65 °C. 5. Run FA 2 from the burette into this flask until a permanent pale pink colour is obtained. 6. Record your titration results in the space provided on page 3. Make certain that your recorded results show the precision of your working. 7. Repeat points 1 to 6 as necessary until consistent results are obtained. 8. Turn off your Bunsen burner.

3    Results [5] (ii) From your titrations, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. Volume of FA 2 =……………………………….. [1] (b) (i) The equation for the reaction between ethanedioate ions and manganate( VII) ions is shown below. 5C2O42–(aq) + 2MnO4–(aq) + 16H+(aq) → 10CO2(g) + 2Mn2+(aq) + 8H2O(l) Calculate the amount, in moles of ethanedioate ions, C2O42– in 25.0 cm3 of FA 1. Amount of C 2O42– in 25 cm3 of FA 1 =……………………………. [1]

4    1 (b) (ii) Determine the concentration, in mol dm–3, of C2O42– in Q. Concentration of C 2O42– in Q =……………………… [2] (iii) Use your answer to (b)(ii) to calculate the M r of the ethanedioate salt. Mr of the ethanedioate salt = …………………….. Hence, deduce the identity of X. Show your working. [Ar: C, 12.0; O, 16.0; H, 1.0; Li, 6.9; Na, 23.0; K, 39.1; Rb, 85.5; Cs, 132.9; Fr. 223.0] X is ………………….. [3]

5    1 (c) A student performed the experiment in (a)(i) using a sample of another ethanedioate salt. The student obtained a mean titre value of 22.20 cm 3. The teacher calculated that the volume of FA 2 required should have been 22.40 cm3. The teacher told the student that the total percentage error from the apparatus in the e xperiment was 0.4 %. Calculate the error in the student's result, based on these data. State and explain whether or not the student's result is accurate. ……………………………………………………………………………………………… ………………............................................................................................................. [2] [Total: 14]

6    2 Evaluation of the reliability of a gas collection method in determining a value for the M r of the ethanedioate salt. In this experiment, you will react the ethanedioate salt with potassium manganate( VII), in the presence of a small amount of manganese( II) ions. You will measure the volume of CO2 gas produced at timed intervals and determine the maximum volume of CO 2 gas produced. FA 4 is a solution containing manganese(II) ions, Mn2+. You will need access to the FA 1, FA 2 and FA 3 solutions you used earlier.           Fig. 2.1 You will measure the level of water in the burette at timed intervals. In an appropriate format in the space provided on page 7, prepare a table in which you may record each burette reading and the time it was taken. In addition, your table will need to show the total volume of CO 2 collected up to that time, recorded to one decimal place. Initial level of tap water 50 cm3 burette Plastic/rubber tubing Large container 250 cm3 conical flask Tap water Glass delivery tube

7    1. Set up the apparatus as shown in Fig. 2.1. You should insert the plastic/rubber tubing to a sufficient depth so that it will not subsequently shake loose. 2. Adjust the water level in the burette until it is between 48.0 cm 3 and 50.0 cm 3. You may find it helpful to use an empty dropping pipette to introduce small amounts of air from the bottom of the burette tube. 3. Use appropriate measuring cylinders to add to the 250 cm3 conical flask.  20.0 cm 3 of FA 1  50.0 cm3 of FA 3 4. Using a dropping pipette, add about 1 cm 3 of FA 4 to the conical flask. 5. Using an appropriate measuring cylinder, measure out 30.0 cm3 of FA 2. 6. Transfer the FA 2 into the conical flask and insert the bung into the conical flask. 7. Allow any bubbles created in the burette when the bung was inserted in the conical flask to rise to the top. Start the stopwatch, read and record the initial water level in the burette. 8. Note: Once you have started the stopwatch, allow it to continue running for the duration of the experiment. You must not stop the stopwatch until the reaction is complete. 9. Check that the plastic/rubber tubing is securely positioned in the burette. 10. Hold the flask by its neck and gently swirl it continuously. 11. At t = 0.5 min, read and record the water level in the burette, to 1 decimal place, together with the time it was measured, in your table. 12. Continue to gently swirl the flask. Read and record the water level in the burette every half minute, until the reaction is complete. (a) (i) Experimental Results [3]

8    2 (a) (ii) Plot on the grid below, a graph of the volume of CO 2 on the y-axis, against time, t, on the x-axis. Draw the most appropriate line, taking into account all of your points.   [4]

9    2 (a) (iii) The equation for the reaction between ethanedioate ions and manganate(VII) ions is shown below. 5C2O42–(aq) + 2MnO4–(aq) + 16H+(aq) → 10CO2(g) + 2Mn2+(aq) + 8H2O(l) Use this equation, and appropriate data