SAJC Prelim P4 Ans

1 H2 Chemistry Preliminary Exam Practical Suggested Solution 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 cm3 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 C2O42- 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.

2 Results Final burette reading/ cm3 25.10 25.10 Initial burette reading/ cm3 0.00 0.00 Volume of FA 2/KMnO4/Titrant / cm3 25.10 25.10 Values used (Tick consistent readings ±0.10 cm3) ✓ ✓ [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. Average volume of FA 2 = (25.10 + 25.10)/2 = 25.10 cm3 Volume of FA 2 =…25.10 cm3…. [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 MnO4– in FA 2 = (25.10/1000) X 0.02 = 0.000502 mol amount of C2O42– in 25 cm3 of FA 1 = 0.000502/2 X 5 = 0.001255 mol Amount of C2O42– in 25 cm3 of FA 1 =…0.001255 mol or 0.00126 mol… [1] 1 (b) (ii) Determine the concentration, in mol dm–3, of C2O42– in Q. amount of C2O42– in 250 cm3 of FA 1 = 0.001255 X 10 = 0.01255 mol [C2O42–]in Q = 0.01255 / (35.70/1000) = 0.3516 mol dm-3 Concentration of C2O42– in Q =…0.3516 or 0.352 mol dm-3 [2] (iii) Use your answer to (b)(ii) to calculate the Mr of the ethanedioate salt. Mr of X2C2O4.H2O = 64.5 / 0.3516 = 183.4

3 Mr of the ethanedioate salt = 183.4 …………… 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] Ar of X = [183.4 – 2(12.0) – 5(16.0) – 2(1.0)]/2 = 38.7 X = Potassium X is K…… [3] 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 experiment 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. Student’s experimental error = 22.40 - 22.20 22.40 × 100% = 0.8928 = 0.893% Since student’s experimental error (0.893%) is more than the apparatus error(0.4%), therefore the student’s result is inaccurate. [2] [Total: 14]

4 2 Evaluation of the reliability of a gas collection method in determining a value for the Mr of the ethanedioate salt. In this experiment, you will react the ethanedioat e 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 CO2 collected up to that time, recorded to one decimal place. 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. Initial level of tap water 50 cm3 burette Plastic/rubber tubing Large container 250 cm3 conical flask Tap water Glass delivery tube

5 2. Adjust the water level in the burette until it is between 48.0 cm3 and 50.0 cm3. 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 cm3 of FA 1  50.0 cm3 of FA 3 4. Using a dropping pipette, add about 1 cm3 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 Time / t / min Burette reading / cm3 (Volume / vol) of CO2 / cm3 0.0 50.0 0.0 0.5 46.5 3.5 1.0 42.0 8.0 1.5 34.5 15.5 2.0 27.0 23.0 2.5 20.2 29.8 3.0 16.5 33.5 3.5 13.8 36.2 4.0 12.5 37.5 4.5 12.0 38.0 5.0 11.6 38.4

6 5.5 11.5 38.5 6.0 11.4 38.6 6.5 11.4 38.6 7.0 11.4 38.6 [3]

7 2 (a) (ii) Plot on the grid below, a graph of the volume of CO2 on the y-axis, against time, t, on the x-axis Vol of CO2 /cm3

8 [4] 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 from your graph, to calculate a value for the amount, in moles of ethanedioate ions, C2O42–, present in 20.0 cm3 of FA 1. [molar volume of gas = 2