SCH 4C Summative - QUANTITATIVE TITRATION - Part 1

Transcription

1 SCH 4C Summative - QUANTITATIVE TITRATION - Part 1 PURPOSE: Design a procedure to make a 0.300mol/L solution of NaOH from solid NaOH pellets. MATERIALS: Solid NaOH Electronic Balance Distilled water Safety goggles 250ml or 500ml volumetric flask with stopper Scrap Paper SAFETY PRECAUTIONS: The NaOH solution is corrosive to skin, eyes and clothing. Do NOT handle the pellets with your skin. Wash any spills or splashes immediately with plenty of water. Wash your hands thoroughly after handling the chemicals. PRE-LAB CALCULATIONS and PROCEDURE: Pure NaOH (s) is often used to create a solution of a known concentration. In order to create the solution from a solid, you must first calculate the amount of pure NaOH to be used. Before you start, you must first outline a step by step method to obtain the proper amount of solid to make a solution of a known concentration knowing your knowledge of quantitative relationships of mass, moles and molar mass. When a solid base (NaOH) is added to distilled water, a solution of a base is created. Concentration is equal to the moles of a substance divided by the liters of the substance as a liquid. C = n / V Today you will have time to write the procedure in rough, and do the procedure to make the solution. You will need to write the procedure again as part of your formal lab write-up. 1. Write the procedure on a separate sheet of paper showing all steps and calculations to be done in order to obtain the proper amount of solid. 2. Write a step by step procedure for mixing the solid with the liquid. Be sure to include a step to label the solution with the experimenters names for use in Part 2.

2 SCH 4C Summative - QUANTITATIVE TITRATION - Part 2 Names: Relative amounts of reactants and products of a reaction are commonly investigated in two ways in the laboratory: gravimetrically (by mass) and volumetrically (by volume and concentration) as in this experiment. Titration is the name given to the process of determining the volume of a solution needed to react with a given mass, or volume, of a sample. We shall use this process to study quantitatively the reaction between an acid and a base. Acid-base titrations involve the neutralization reaction between aqueous hydrogen and hydroxide ions. These ions combine to form the neutral water molecule: H + (aq) + OH (aq) H 2 O (l) Recall that indicators may be used to determine of the hydrogen ion concentration. The point at which an indicator changes colour is called the endpoint, indicators change colour at a specific ph. Phenolphthalein will be used as the indicator in this experiment since its colour change occurs when an equal number of moles of acid and base have been added. This point in the reaction is called the equivalence point. At neutralization the concentration of H + and OH are the same and the equivalence point has been reached. The total number of moles of acid or base reacting can be calculated by manipulating the following equations: C = n / V or V a C a = V b C b. Vinegar is a dilute solution of acetic acid, CH 3 C OH. Only the hydrogen atom that is attached to an oxygen atom is acidic. Thus, acetic acid contains only one acidic hydrogen. As a consumer, you can buy vinegar with different concentrations. For example, the concentration of table vinegar is different from the concentration of the vinegar that is used for pickling foods. To maintain consistency and quality, manufacturers of vinegar need to determine the percent concentration of acetic acid in the vinegar. In this investigation, you will determine the molarity of acetic acid in a sample of vinegar. In addition, you will calculate the percentage, by mass, of the vinegar. The mass of the acetic acid (CH 3 COOH) in the vinegar is obtained from a mole-mass calculation. The mass of the solution is obtained from the measured volume of the solution times its density (D = m / V ). For this experiment, you may assume that the density of the vinegar is 1.01 g / ml. Titration is one of the most common operations performed by the chemist. We all depend upon chemical analysis, and it is with this branch of chemistry that the average citizen is most likely to come into contact. Decisions involving huge sums of money, or even life and death, depend upon the accuracy and speed of chemical analysis, whether in hospital lab testing, environmental pollution monitoring, or crime detection.

3 QUANTITATIVE TITRATION PURPOSE: To determine the concentration and percent composition of a sample of vinegar. HYPOTHESIS: Make a guess as to the molarity of vinegar and its percent composition by mass. MATERIALS retort stand burette funnel safety goggles burette clamp 250 ml beaker 250 ml Erlenmeyer flask phenolphthalein distilled water vinegar M NaOH (aq) SAFETY PRECAUTIONS The NaOH solution is corrosive to skin, eyes and clothing. Wash any spills or splashes immediately with plenty of water. Phenolphthalein is poisonous and flammable. Do not get any in your mouth; keep well away from flame. The vinegar solutions are mildly corrosive. Keep them off your skin and out of your eyes. Wash any spills and splashes immediately with plenty of water. PROCEDURE 1. See the section on the care of burettes at the end of this experiment. Set up a retort stand, burette clamp, burette, and funnel. Rinse the burette first with distilled water. Then rinse it with 10 ml sodium hydroxide solution. After rinsing the burette, fill it with the base. Make sure that there are no air bubbles in the burette. Also make sure that the liquid fills the tube below the glass tap. Remove the funnel before beginning the titration. Record the volume in the burette by reading the bottom of the meniscus to the nearest 0.01 ml. 2. Go to the burette set up at the side of the room and add ml of vinegar into a 250 ml Erlenmeyer flask. Record this amount. Add approximately 15.0 ml of water. Also add half an eyedropper of phenolphthalein indicator. 3. Place a sheet of white paper under the Erlenmeyer flask. Hold the neck of the Erlenmeyer flask with one hand and manipulate the burette with the other. As you add the sodium hydroxide, gently swirl the flask so the solutions will become mixed. Continue adding sodium hydroxide until the first faint pink colour develops. If the colour disappears upon mixing the solution, add more sodium hydroxide, drop by drop, until a persistent pale pink colour is obtained. If you are not sure whether you have reached the end-point, take the burette reading. Add one drop of sodium hydroxide, or part of a drop. Observe the colour of the solution. (Take care not to go beyond the last calibration marks on the burette.) Record the volume reading at the bottom of the meniscus of the burette. Rinse the Erlenmeyer flask thoroughly before repeating the titration. 4. Repeat the titration until you obtain volumes of base that are within 0.5 ml of each other. 5. When you have finished, dispose of the chemicals as directed. Rinse the pipette and burette 3 times each with water. Leave the burette tap open. REAGENT DISPOSAL Mix any leftover acids and bases together to neutralize, and pour down the sink with plenty of water. Do not return any solutions to their original containers.

4 DATA AND OBSERVATIONS Determination of Concentration and Percent Composition of Vinegar Concentration of NaOH Trial 1 Trial 2 Trial 3 Trial 4 Volume of vinegar (ml) Initial volume of NaOH (ml) Final volume of NaOH (ml) Volume of NaOH used (ml)

5 CALCULATIONS 1 a) Write the balanced chemical equation for the reaction of acetic acid with sodium hydroxide. b) According to the equation, how many moles of acetic acid are used per mole of base? c) Using the relationship in b), calculate the moles of acetic acid used. 2. a) Calculate the molarity of acetic acid in your vinegar sample. Use the average volume and concentration of sodium hydroxide, and the volume of vinegar. 3. a) Find the molar mass of acetic acid. b) Calculate the mass of acetic acid in the volume of vinegar you used. c) The density of vinegar is 1.01 g / ml. Calculate the mass of the vinegar sample. d) Find the percent, by mass, of acetic acid in the vinegar sample.

6 Discussion 1. a) Explain the difference between the equivalence point and endpoint of a titration. b) What was the reason for rinsing out the burette with NaOH solution before starting the titrations? c) While doing a titration, it is permissible to use a wash bottle of distilled water to wash down any material that may have splashed higher up. This would appear to increase the volume of the acid in the flask. Why will it have no effect on the results? 2. a) By law, vinegar must be not less than 4% by mass acetic acid. Did your sample meet this specification? Explain. b) List several possible sources of error in the experiment. 3. Most shampoos are basic. Why do some people rinse their hair with vinegar after washing it? Conclusions: One sentence please.

7 CARE OF BURETTES Cleaning Place a few milliliters of a detergent solution into a burette, then use a burette brush to clean the inside surface. Rinse well, first with tap water and then with distilled water. After draining the burette, note if there are any droplets still adhering to the sides of the tube. If there are, the glass is not thoroughly cleaned and should be rewashed. When glass is clean, water wets it evenly. Preparing for Use After cleaning a burette, add 5 10 ml of the solution that is to be used in that burette. Let several milliliters of solution flow through the tip. Turn the burette to a horizontal position and with a rotary motion, slowly pour the rest of the solution out of the top. Make sure that the solution wets the inside completely. For a more complete rinsing, repeat the above. Fill the, burette to the top with the solution to be used. Let solution flow from the tip until the bottom of the meniscus is at zero or below. This should eliminate air bubbles in the tip. If a drop hangs on the tip before you start a titration, discard it by touching it to a beaker. However, a drop formed during a titration must be caught by touching it to the side of the container being used and rinsing it into the container with distilled water. Reading the Volume When reading the volume on the burette, be sure to have your eye level with the bottom of the meniscus and read the volume carefully at the bottom of the curve. In each titration use an absolute minimum of 10 ml of each solution to attain a precision of 1.0 %. After Use Drain and rinse the burette several times with tap water, then, as a final rinse, use distilled water. Glass reacts with basic solutions so take special care in rinsing a burette that has had such solutions in it. A rinse with dilute acid after one water rinse will help assure that the base is removed. Follow this with the water rinses described above. Leave the stopcock open when storing the burette.

8 Quantitative Lab Rubric Student Name: Overall Expectation Demonstrate an understanding of the basic principles of qualitative analysis and underlying theories Carry out qualitative analysis, using flow charts and appropriate laboratory equipment instruments Describe the role and importance in society of some of the applications of qualitative analysis.?guiding Question? Have I clearly demonstrated my knowledge of qualitative analysis and related that to theories that explain what I observed? Have I logically and clearly followed qualitative observations through to interpret the observation and make conclusions? Have I related what I have observed and the principles learned to useful applications in real life? Level 4 To an exceptional degree Level 3 To a considerable degree Level 2 To a moderate degree Level 1 To a limited degree Not Yet Demonstrate an understanding of the mole concept as well as of quantitative relationships in chemical reactions Use techniques of quantitative analysis in the preparation of standard solutions, and solve problems involving the analysis of quantities in chemical reactions, using both theoretical and experimentally measured quantities Explain the importance of quantitative chemical relationships in industry and in everyday life. Have I clearly shown proper manipulation of mole and hub calculations to obtain useful data from my quantitative observations? Have I properly prepared a standard solution using proper technique? Have I followed all instructions correctly to use the apparatus and materials in the experiment? Have I related the calculations and/or procedure to an industrial or commercial application? Use the techniques involved in the quantitative analysis of solutions effectively and accurately; Have I observed, recorded, analyzed, and communicated results of an inquiry with a high degree of clarity? Mark: You are to include the following in your final word processed lab report in 12 point Times New Roman Font including Title Page: Procedure for finding the proper amount of NaOH to mix Procedure to mix the solution of NaOH Observations of your titration Calculations showing all intermediary work Answers to discussions in typed format A Conclusion Bibliography of any research you completed