THE RACI TITRATION STAKES 2018

Transcription

1 THE ROYAL AUSTRALIAN CHEMICAL INSTITUTE INCORPORATED THE RACI TITRATION STAKES 08 INSTRUCTIONS FOR TEAM MEMBERS THE CHALLENGE Vinegar is used to prepare many different kinds of sauces. The main constituent of any kind of vinegar is ethanoic acid, CH COOH, which is called acetic acid on the labels of commercial products. This gives vinegar its sharp taste. The Vinegar and Sauce Company is in a fine pickle! It needs to know the concentrations of three different samples of ethanoic acid, 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! Each member of your team will be performing two sets of acid-base titrations to determine the concentration of one of the three samples of ethanoic acid. THE PLAN FOR THE ANALYSIS Now CH COOH is a weak acid. To standardise (determine the precise concentration of) a CH COOH solution, you need to titrate it against a solution of a strong base of precisely known concentration. You will use NaOH solution for that purpose. The catch is, being a strong base, NaOH solution absorbs acidic oxides such as CO out of the air. So its concentration in a solution cannot be known ahead of time and must be worked out first, as quickly as possible after the bottle is opened before it absorbs any more CO. For that reason, the NaOH solution will first be standardised against the strong acid HCl. This solution will be supplied to you at the precisely known concentration of mol L -. The balanced equation for the first set of titrations is HCl(aq) + NaOH(aq) NaCl(aq) + H O(l) The balanced equation for the second set of titrations is CH COOH(aq) + NaOH(aq) CH COONa(aq) + H O(l)

2 The plan is summarised in Figure, on page. Figure. The plan for the analysis. The two sets of titrations you will perform will require you to achieve as precise results as possible. The end-point is the point at which drop of the solution being delivered by the burette causes the indicator to change colour. We assume that is indicating when the acid and base have exactly reacted with each other. As you will always have the NaOH in the burette, and the indicator you will use is phenolphthalein, the end-point will occur when just more drop of NaOH solution turns the solution from colourless to a permanent purple-pink colour. (Just before that, you will see a pink colour but it will disappear on swirling the solution.) The volume of solution delivered by the burette to reach the end-point is called the titre. If you have titres within 0.0 ml of each other, they are described as concordant results. They are the only ones used to calculate the unknown concentration. Figure on page shows the titration process you will use each time. Achieving concordant results As shown in Figure, your results will be more accurate if you can achieve at least concordant results. This is what to do if you do not have enough time to obtain concordant results.. If you only obtain concordant results, use these two results to determine the mean titre.. If you perform two accurate titrations but the titres are not within 0.0 ml of each other, use the average of these two titres for your calculations.. If you only perform one accurate titration, use this result for your calculations.

3 Figure. Performing the titrations. In the first set of titrations the acid delivered by pipette is the M HCl solution. In the second set, the acid you will use is the CH COOH solution, as shown in Figure. YOUR TEAM Your teacher will place you in a team of three. All team members will need to work together very efficiently to complete the challenge in time. You will be judged by your team results, not by your individual results.

4 SAFETY WARNING Before you start the challenge, make sure you are all wearing fully protective clothing with no loose sleeves, and safety glasses. Tie back hair that touches your shoulders. Only use a pipette filler to draw solutions up into the pipette. There must be no eating or drinking in the laboratory. GETTING STARTED THE MATERIALS YOU NEED Checklist of chemicals and equipment per team bottles M HCl solution bottle NaOH solution A bottle NaOH solution B bottle NaOH solution C bottle CH COOH solution A bottle CH COOH solution B bottle CH COOH solution C x dropper bottles phenolphthalein solution x clean dry burettes, labelled A, B and C x burette stands 6 x 0.00 ml clean, dry pipettes ( labelled A, labelled B and labelled C) x pipette fillers that fit 0.00 ml pipettes 9 x 00 ml clean, dry beakers labelled NaOH, labelled HCl, labelled CH COOH 9 x 50 ml clean, dry conical flasks x medium-sized clean, dry glass funnels x white ceramic tiles x wash bottles filled with de-ionised water Paper towel

5 SETTING UP THE EQUIPMENT Step. Set up the three burettes in their stands, with the white tile below the burette. Step. Spread paper towel out on the bench. Step. Next to the burette labelled A, place the following on the paper towel. x bottle of HCl solution, x bottle of NaOH solution A x bottle of CH COOH solution A x dropper bottle phenolphthalein x wash bottle x 0.00 ml pipettes x pipette filler that fits a 0.00 ml pipette x small beaker labelled NaOH x small beaker labelled HCl x small beaker labelled CH COOH x glass funnel x 50 ml conical flasks Step. Repeat Step for the other two burettes, with items marked B next to burette B and items marked C next to burette C. These will now be called Station A, Station B and Station C. Step 5. At each station, close the tap of the burette. Pour a little of the NaOH solution into the small beaker labelled NaOH. Now tip this into the burette using the funnel. Place the beaker under the burette. Make sure the burette does not leak. Swirl the solution around and drain into the beaker. Discard the solution down the sink. Do not rinse the beaker or funnel out with water! They will be used for filling the burette for all your titrations. Step 6. At each station pour a little of the HCl solution into the small beaker labelled HCl. Making sure the end of the pipette is below the surface of the solution, draw some of this HCl solution up into one of the pipettes until it is partly up the bulb, using the pipette filler. Swirl the solution around then empty the pipette over the sink. Make sure that none of the solution enters the pipette filler! This pipette will only be used for the HCl solution. The other pipette will only be used for the CH COOH solution. Ensure they do not get mixed up. 5

6 THE COMPETITION BEGINS! Now that your equipment is ready, it is time to start the competition. 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. You have 90 minutes to perform all your titrations and calculations and complete the results sheet. You will be given 0 minutes warning before the end. THE EXPERIMENTAL PROCEDURE FOR EACH STATION Titrations Set Standardising the NaOH solution Step. Step. Step. Step. Step 5. Step 6. Pour about 50 ml of the NaOH solution into the NaOH beaker. After closing the burette tap, carefully pour the NaOH solution into the burette using the funnel until it reaches just below the zero mark. Now place the beaker under the burette and slowly let a little solution through so that the stem of the burette below the tap now contains solution, then close the tap. Do not attempt to add more NaOH to make up the volume to the zero mark! This would be a waste of precious time! Remove the funnel. Pour about 50 ml of the HCl solution into the HCl beaker. Making sure the end of the pipette is below the surface of the solution, draw some of this HCl solution up into one of the pipettes until it is just above the etched mark, using the pipette filler. Holding the pipette above the beaker, now slowly let some solution out until the bottom of the meniscus just sits on the etched mark. Transfer the HCl to one of the conical flasks and let it drain into the flask. Do not attempt to shake out or blow out the last drop! The etched mark already takes the volume of that drop into account. Add 5 drops of phenolphthalein. Place the flask under the burette, raising or lowering the burette so that its tip just reaches the bottom of the vertical part of the flask. Record the initial volume of the NaOH in the results table on page 8. Slowly add the NaOH to the flask with swirling, until drop changes the solution from colourless to a purple-pink that does not disappear on swirling. Occasionally rinse the sides of the flask with a little water using the wash bottle to ensure all the solution can react. Record the final volume of the NaOH in the same results table. Repeat Steps to 5 as many times as required and as time allows, except you will not need to fill the burette stem below the tap each time. It will already be full. Always use the same top section of the burette, from just below the zero mark, as this will reduce instrumental error and increase your chance of obtaining concordant results. It also will avoid running out of solution before the end-point is reached. 6

7 At this point, you may wish to calculate the concentration of the NaOH in bottles A, B and C. See the advice on page regarding which results should be used for your calculations. Each answer will be different, but we have been advised that in each case [NaOH] is somewhere between M and 0.00 M. If you get results outside this range, something has gone wrong. Most probably there is an error in your calculations, so check them, including your unit conversions. Titrations Set Standardising the CH COOH solution This procedure will be the same as for Titrations Set, except that the acid used will be CH COOH, and your results should be recorded in the table on page 9. Note:. Thoroughly wash the three conical flasks with water then rinse with de-ionised water before you start.. Ensure that the pipette and beaker used for HCl are rinsed over the sink then removed from the station so they do not contaminate the CH COOH. You are now able to complete your calculations. CALCULATION GUIDE Example: Standardising the NaOH solution See the equation for this reaction on page. All answers should be given to significant figures. The amount of HCl in mol in each titration is given by n(hcl) = c x V where c = concentration in mol L - = x V = volume = 0.00 ml = L = mol From the equation, since the mole ratio NaOH : HCl = : n(naoh) = n(hcl) = mol Now c(naoh) = n/v where V = mean titre = ml = L Hence [NaOH] = mol L - 7

8 GROUP RESULTS Titrations Set Note: All volumes must be recorded to decimal places. Use the key to mark the results used to calculate each mean titre. NaOH solution Run Initial burette reading Final burette reading Titre A Mean titre for 0.00 ml M HCl B Mean titre for 0.00 ml M HCl C Mean titre for 0.00 ml M HCl Key * = results used to determine mean titre 8

9 Titrations Set Note: All volumes must be recorded to decimal places. Use the key to mark the results used to calculate each mean titre. NaOH solution Run Initial burette reading Final burette reading Titre A Mean titre for 0.00 ml CH COOH solution A B Mean titre for 0.00 ml CH COOH solution B C Mean titre for 0.00 ml CH COOH solution C Key * = results used to determine mean titre 9

10 GROUP CALCULATIONS Titrations Set : Standardising the NaOH solutions From page 7, n(hcl) used each time = mol NaOH Solution A n(naoh) = n(hcl) = mol Now c(naoh) = n/v where V = mean titre = ml = L Hence [NaOH] = mol L - NaOH solution B n(naoh) = n(hcl) = mol Now c(naoh) = n/v where V = mean titre = ml = L Hence [NaOH] = mol L - NaOH solution C n(naoh) = n(hcl) = mol Now c(naoh) = n/v where V = mean titre = ml = L Hence [NaOH] = mol L - Check! Is each [NaOH] is somewhere between M and 0.00 M? Is each [NaOH] given to significant figures? 0

11 GROUP CALCULATIONS continued Standardising the CH COOH solutions See the equation for this reaction on page. The mole ratio CH COOH : NaOH = Hence the mole relationship is n(ch COOH) = x n(naoh) CH COOH solution A [NaOH solution A] = mol L - Mean titre of NaOH solution A = ml = L n(naoh) = c x V = x = mol Hence n(ch COOH) = mol V(CH COOH) = 0.00 ml = L Hence [CH COOH] = n / V = mol L - CH COOH solution B [NaOH solution B] = mol L - Mean titre of NaOH solution B = ml = L n(naoh) = c x V = x = mol Hence n(ch COOH) = mol V(CH COOH) = 0.00 ml = L

12 CH COOH solution C [NaOH solution C] = mol L - Mean titre of NaOH solution C = ml = L n(naoh) = c x V = x = mol Hence n(ch COOH) = mol V(CH COOH) = 0.00 ml = L Hence [CH COOH] = n / V = mol L - Check! Each [CH COOH] should be somewhere between M and 0.00 M? Are yours? Check your calculations if not. Is each [NaOH] given to significant figures? The sheet you submit to the RACI electronically is on page. Be very careful when you copy the required information from your results tables and calculations! Double-check you have done it correctly. And double-check that the names of all your team members are spelled correctly. 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!

13 RACI TITRATION STAKES 08 - RESULT SHEET SCHOOL NAME TEAM MEMBERS TITRATION RESULTS Identity of NaOH solution NaOH solution A NaOH solution B NaOH solution C Volume of M HCl solution * Mean titre of NaOH solution * This should be 0.00 ml in most cases. Identity of NaOH solution Volume and identity of CH COOH solution * NaOH solution A CH COOH solution A: NaOH solution B CH COOH solution B: NaOH solution C CH COOH solution C: * This should be 0.00 ml in most cases. Mean titre of NaOH solution CONCENTRATIONS OF SOLUTIONS Solution Concentration NaOH solution A mol L - NaOH solution B mol L - NaOH solution C mol L - CH COOH solution A mol L - CH COOH solution B mol L - CH COOH solution C mol L -