TITRATION STAKES 2018 INSTRUCTIONS TO TEAM MEMBERS

Transcription

1 THE ROYAL AUSTRALIAN CHEMICAL INSTITUTE INCORPORATED TITRATION STAKES 2018 INSTRUCTIONS TO TEAM MEMBERS (A copy of these instructions is to be given to every participant before commencement of the competition. All team members must read these instructions carefully before starting.) INTRODUCTION The Vinegar and Sauce Company is in a pickle! It needs to know the concentrations of three samples of ethanoic acid, commonly known as acetic acid or vinegar, labelled A, B and C, for a new range of products it is developing, but does not have the resources to do the work itself. School students participating in this year s titration competition are being asked to help! The concentration of an acid solution can be determined by titration with a solution of an alkali or base of known concentration. Unfortunately, the solutions of sodium hydroxide (caustic soda), also labelled A, B and C, which have been supplied are also of unknown concentration. The only solution which is of known concentration is the hydrochloric acid solution, which is known to be exactly Molar. What you are being asked to do is: (a) to determine the concentrations of the three sodium hydroxide solutions by titrating with hydrochloric acid solution; then, (b) to determine the concentrations of the three ethanoic acid solutions by titrating with the sodium hydroxide solutions. To make the experiments simpler, sodium hydroxide solution A should be titrated with ethanoic (or acetic) acid solution A, sodium hydroxide solution B should be titrated with ethanoic acid solution B and sodium hydroxide solution C should be titrated with ethanoic acid solution C. What we have been told is that the concentrations of the three sodium hydroxide solutions and the three ethanoic acid solutions are all within the range and Molar. If you get results outside this range, Page 1 of 7

2 something has gone wrong probably the arithmetic so please CHECK YOUR CALCULATIONS! INSTRUCTIONS Before everyone starts, ensure that every member of your team is wearing appropriate protective clothing safety glasses, lab coat and enclosed shoes. Each team comprises three members and this year we want members of each team to work together on all samples so that everybody contributes towards the team result. We are looking for a team outcome rather than individual results. A. PRELIMINARIES 1. Check that your team has three bottles of hydrochloric acid (which will all be identical), three bottles of sodium hydroxide solution labelled A,B and C and three bottles of ethanoic (or acetic) acid labelled A,B and C. The total volume of hydrochloric acid used by the team will be of the order of 350 to 400 ml. The sodium hydroxide bottles each contain approximately 250 ml of sodium hydroxide solution, and the ethanoic (or acetic) acid bottles each contain approximately 125 ml. (Teams need twice as much sodium hydroxide solution as hydrochloric acid or ethanoic acid as the sodium hydroxide is used twice both for the titration with hydrochloric acid and with the acetic acid.) The solutions will be supplied by the RACI. The School will supply the following materials and equipment: Phenolphthalein indicator solution Droppers and Bulbs Conical Flasks or Beakers for the titration (six per team) Small Beakers for filling burette or pipetting solutions (six per team) Small funnels for filling burette (three per team) Page 2 of 7

3 Washbottles filled with distilled or de-ionized water 25 or 20 ml pipettes (six per team) 50 ml burettes (three per team) Pipette Fillers (three per team) 2. Rinse your apparatus with distilled water and make sure the burette does not leak. If you cannot dry your pipettes and burettes before doing the experiments, rinse each one with a small quantity of solution to be used in it DO NOT TOUCH THE SOLUTION BOTTLES UNTIL YOU ARE ADVISED TO DO SO. 3. ONCE YOU HAVE BEEN TOLD TO BEGIN AND UNTIL THE END OF THE COMPETITION, THERE MUST BE NO COMMUNICATION WITH MEMBERS OF OTHER TEAMS. YOU MAY ASK FOR ADVICE AND GUIDANCE FROM A SUPERVISOR. YOU WILL NEED TO COMMUNICATE AND WORK CLOSELY WITH OTHER MEMBERS OF YOUR TEAM. 4. From the time you are told to start, you have 90 minutes to complete all titrations and calculations and to complete the Result Sheets. You will be given about ten minutes warning before the end of your time. B. DETAILED INSTRUCTIONS FOR EACH TEAM 1. Set up three burettes per team and mark them A, B and C. CAREFULLY transfer sodium hydroxide solution A into burette A, sodium hydroxide solution B into burette B and sodium hydroxide solution C into burette C. Do not overfill! Read the levels in each burette accurately. Make sure that you do not mix up the sodium hydroxide solutions! (Hint: keep the sodium hydroxide solution marked A close to the burette marked A, the solution marked B close to the burette marked B, and solution C close to burette C.) Page 3 of 7

4 2. Pipette 20 or 25 ml of hydrochloric acid into each of three conical flasks or beakers. Note that you should be very careful to ensure that the pipette is clean and dry or has been rinsed through with the hydrochloric acid solution before use. Make sure you follow the correct technique of pipetting. (The hydrochloric acid is of known strength, and the purpose of this part of the exercise is to determine the concentrations of the sodium hydroxide solutions (A, B and C). You do this by measuring how much sodium hydroxide solution is needed to neutralise the acid in the flask or beaker i.e. how much sodium hydroxide solution is needed to reach the end-point when the indicator changes colour.) 3. Add up to five drops of indicator to each conical flask or beaker. 4. Put one flask or beaker under each of the three burettes and titrate the sodium hydroxide solution from the burettes carefully and slowly into each flask or beaker until the colourless solution just turns a permanent faint pink colour. NOTE: As the concentration of each of the sodium hydroxide solutions is different, the titres from the A, B and C burettes will be different. It is likely that your first titrations will overshoot the end-point. This will help you to know when to reduce the rate of addition of the sodium hydroxide solution for the second and subsequent titrations. You may carry out the experiment several times to get a consistent end-point. Record what you believe are the most accurate volumes of the three sodium hydroxide solutions (A, B and C) needed to neutralise the acid. Remember, you will probably need to top up the burette after each titration: remember to record the levels carefully and DO NOT MIX THE SOLUTIONS. 5. Now, into three clean conical flasks or beakers, pipette 20 or 25 ml of acetic (ethanoic) acid solution. Remember to use a clean, dry pipette. The flask containing ethanoic acid solution A should be placed under burette A (containing sodium hydroxide solution A) the flask containing ethanoic acid solution B should be placed under burette B (containing sodium hydroxide solution B) and the flask containing ethanoic acid solution C should be placed under burette C. IT IS IMPORTANT NOT TO GET THE SOLUTIONS MIXED UP! Now add up to five drops of indicator solution and run in the sodium hydroxide solutions from the burettes until the colourless solution just starts to turn a Page 4 of 7

5 permanent pink colour. Remember that your first titration is a sighter and that you will probably overshoot the end-point. Use this information to control the rate of addition of the sodium hydroxide solution in subsequent titrations as you get near to the end-point. Again, repeat the titration (remembering to top up the burette each time with the correct concentration of sodium hydroxide solution) until you are confident that you have an accurate figure. 6. RECORDING THE MEASUREMENTS AND CALCULATING THE RESULTS: The information you will need to submit to the RACI electronically is as follows: Your school name: The names of the members of your team: Your titration volumes (best values) to two decimal places: Identity of NaOH solution NaOH solution A NaOH solution B NaOH solution C Volume of Hydrochloric acid (20 or 25 ml) Volume of NaOH solution needed to neutralise the HCl solution (ml) Identity of NaOH solution Volume and identity of ethanoic acid (20 or 25 ml) NaOH solution A Ethanoic Acid A: NaOH solution B Ethanoic Acid B: NaOH solution C Ethanoic Acid C: Volume of NaOH solution needed to neutralise the acetic acid (ml) Your calculations: A. Calculate the molarity of your sodium hydroxide solutions to four significant figures: Page 5 of 7

6 For each sodium hydroxide solution, molarity equals ratio of the volume of sodium hydroxide (from the burette) to the volume of hydrochloric acid (from the pipette) multiplied by the molarity of the HCl (which is given as M). (Example: If you pipetted ml of acid into the flask and had to add ml of sodium hydroxide solution from the burette, the molarity of the sodium hydroxide solution is divided by times M (= M). Remember, it you have to run in a greater volume of sodium hydroxide solution from the burette than the volume of acid you pipetted into the flask, the sodium hydroxide solution is LESS CONCENTRATED than the acid solution. If you have to run in less alkali to reach the end-point, then the sodium hydroxide solution is MORE CONCENTRATED. Use this as a check so that you don t fall into the trap of putting the ratio the wrong way round!) B. Calculate the molarity of your ethanoic acid solutions to 4 significant figures: For each acetic acid solution, as above, molarity equals the ratio of the volume of sodium hydroxide solution (from the burette the concentration of which you have just determined) to the volume of acetic acid (from the pipette). Remember NOT TO MIX UP THE SOLUTIONS SODIUM HYDROXIDE SOLUTION A SHOULD BE USED TO NEUTRALISE ACETIC (ETHANOIC) ACID SOLUTION A, SODIUM HYDROXIDE SOLUTION B SHOULD BE USED TO NEUTRALISE ACETIC (ETHANOIC) ACID SOLUTION B, AND SODIUM HYDROXIDE SOLUTION C SHOULD BE USED TO NEUTRALISE ACETIC (ETHANOIC) ACID SOLUTION C. (Example: If you have pipetted ml of ethanoic acid into the flask and had to add ml of sodium hydroxide solution from the burette to reach the end point, the molarity of the acetic acid is divided by multiplied by the NaOH molarity which has been determined to be M. The concentration of the acetic (ethanoic) acid is, therefore, M. The ethanoic acid in this case is less concentrated than the sodium hydroxide solution because you had to add less alkali solution to reach the end-point.) NOTES: Page 6 of 7

7 The volumes of solutions supplied should be sufficient to allow three or four titrations to be carried out on each set of solutions, provided there are no major accidents. If breakages or spillages occur, report them to your supervisor. In some circumstances solutions or apparatus may be replaced. Make sure you have enough time to do your calculations. As a Team, go through your workings very carefully in order to avoid major mistakes (such as the decimal point in the wrong place). The results are to be submitted on line. Your teacher will advise how this will be done. At the end of the competition, rinse your apparatus with water and follow any other directions of supervisors regarding cleaning up. Remember that all spillages MUST be cleaned up as soon as possible to avoid slip hazards. The RACI hopes you have enjoyed participating in this year s competition. The Vinegar and Sauce Company is very grateful for all the help you are giving them! Page 7 of 7