Titration of a strong base with a strong acid

Transcription

1 Experiment 3 Titration of a strong base with a strong acid Aim The aim of this experiment is to determine the concentration of an aqueous sodium hydroxide solution. Introduction In this experiment you will determine the concentration of an aqueous sodium hydroxide solution. This solution is titrated with a standard solution of dilute hydrochloric acid. Methyl red is used as the ph indicator. It shows a red colour below ph = 4.8, and a yellow colour above ph = 6.0. Procedure 1. Background Read the sections dealing with the use of the pipette and burette in your manual. Before performing the titrations yourself, the supervisor will demonstrate to you the proper use of the volumetric glassware. The difference between a rough titration and an accurate titration will also be demonstrated. 2. Preparation of glassware (a). Make sure that all glassware used in the experiment is clean and not greasy: no droplets of water should stick to the walls of the pipette and the burette. If necessary, wash the glassware with soap or detergent solution. (b). Rinse the glassware with tap water and then with distilled water. 3. Collection of solutions to be used in the experiment Use clean 100 ml beakers. Label them clearly with the name of the solution. Before you add the stock solution to your beaker make sure that the beaker is dry, or rinse the beaker with a small quantity of the stock solution. Note: NEVER pour excess solution back into the stock bottle. Page 1

2 4. Filling of the burette with aqueous hydrochloric acid (a). Rinse the burette and a small funnel twice with some 10 ml of aqueous hydrochloric acid. (b). Clamp the burette and fill it with dilute hydrochloric acid till close to the zero mark. Use a small funnel and remove it immediately after use. (Make sure that air can escape from the burette when filling it.) (c). Drain a few milliliters of the solution to make sure that there are no air bubbles trapped in the jet below the tap. If the air bubble persists, slant the burette at the sink and open the tap to allow some of the solution to flow out with the air bubble. (d). Take the initial burette reading to 0.02 ml. To avoid parallax error, your eye should be level with the bottom of the meniscus when you take this reading. 5. Transfer of aqueous sodium hydroxide solution to the conical flask (a). Pour about 5 ml of the aqueous sodium hydroxide solution into a clean beaker. Using this solution, rinse the pipette twice. (b). Discard the solution left in the beaker (if any) and refill with enough fresh aqueous sodium hydroxide solution needed for the actual transfer. (c). Pipette 10 ml of aqueous sodium hydroxide solution into a conical flask: # Use a pipette-filler to fill the pipette above the mark. # Remove the bulb and use your index finger to bring the level of liquid to the line on the pipette. Make sure that the lower curve of the meniscus sits on the line. # Transfer the liquid to a clean conical flask. (Note: The conical flask should have been rinsed with distilled water only). # Do not blow out the small amount of liquid that remains in the pipette. # Place the conical flask on a white tile. (Refer to pages 5 and 6 in the Introduction). (d). Using a wash bottle with distilled water, flush down the droplets of the aqueous sodium hydroxide solution remaining on the inside wall of the flask. Page 2

3 6. Titration (a). Titration technique (i) Hold and control the burette tap with the left hand, at the same time swirl the conical flask with the right hand. (If you are lefthanded the opposite applies). Refer to the diagram on the left. (ii) Rinse down the sides of the conical flask with distilled water during the titration. Learn how to control the tap to give you a drop at a time and also how to split a drop so that you can add half a drop near the end point. (iii) Perform one rough titration followed by two accurate titrations. (This should be carefully demonstrated). (b). Rough titration A 'rough titration is normally performed as a time-saving procedure if sufficient sample is available. The accurate titrations can then be performed much faster. Steps: (i). Add a few (1 or 2) drops of methyl red indicator solution to the aqueous sodium hydroxide solution inside the conical flask. Swirl the flask. Note the colour. (ii). Place the conical flask on a white tile. (iii). Record the initial reading of the burette in the table of your report sheet. (iv). From the burette, add 1 ml portions of the dilute hydrochloric acid at a time, to the conical flask. Swirl the flask carefully after each addition, but not too vigorously! (v). Stop the titration when the yellow colour has become orange and record your final reading. At the endpoint of the titration the indicator has just changed colour. Page 3

4 (c). Accurate titration From the rough titration you know where approximately the indicator changes colour (i.e. where the end point is). Therefore now a large volume of the titrant can be added all at once. Then a drop at a time is added close to the endpoint. (i). Clean the conical flask with tap water and then rinse it with distilled water. (ii). Transfer 10 ml of the aqueous sodium hydroxide solution with the bulb pipette to the conical flask. Add a few drops of indicator and place it on the white tile. (iii). From the burette add a large volume of dilute hydrochloric acid, i.e. 1 to 2 ml less than was needed for a colour change of the indicator in the rough titration. Swirl the conical flask. (iv). Now add dilute hydrochloric acid drop by drop, until the indicator starts to get a permanent change in colour. Swirl the flask carefully after each addition. (v). Record your final reading of the burette (to 0.02 ml) in your Report sheet. (vi). Repeat the accurate titration once more. Two accurate titrations volumes must be within 0.10 ml. If not, repeat the titration as many times as required. Page 4

5 Surname: First names: Student No.: Group: Experiment 3: Pre-lab sheet Complete this sheet before you come to the practical; have it signed by the demonstrator. 1. Write down the apparatus and chemicals required in this experiment. Apparatus required Chemicals required 2. To how many decimal places should you record (a). the volume (in ml) delivered by the 10 ml pipette? (b). the readings (in ml) of the burette? 3. Explain the following practices in the titration procedure: (a). The burette and the pipette are each rinsed with distilled water and finally each is rinsed with some of the solution that will be measured or transferred. (b). The conical flask is only rinsed with distilled water. Page 5

6 4. During the titration distilled water is used to wash down the inside of the conical flask. Why is this done and why does adding water here not affect the titration results? 5. The results of a titration experiment were as follows: # Volume of aqueous NaOH pipetted into the conical flask = ml # Average volume of dilute HCl added from the burette to reach the endpoint = ml # Molarity of the dilute HCl = M (a). Write the chemical reaction equation for the titration reaction. (b). Calculate the number of moles of HCl added during the titration. (c). What is the number of moles of NaOH that has reacted during the titration? (d). What is the molarity of the aqueous NaOH? Page 6

7 Surname: First names: Student No.: Group: Experiment 3: Report sheet This report sheet is to be submitted before leaving the lab. Experimental data Volume of aqueous sodium hydroxide used = Concentration of aqueous hydrochloric acid = Burette reading Final reading / ml Rough titration Accurate titrations 1 st 2 nd 3 rd (if necessary) Initial reading / ml Volume added / ml Average volume of aqueous hydrochloric acid added = Calculations 1. Write the chemical equation for the reaction between aqueous sodium hydroxide solution and aqueous hydrochloric acid. 2. Calculate the amount (number of moles) of HCl (aq) added during the titration. Page 7

8 3. What is the amount (number of moles) of NaOH (aq) that has reacted? 4. Calculate the concentration (mol L -1 ) of the aqueous sodium hydroxide solution. 5. Give in one sentence the conclusion of your experiment. Page 8