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