Percentage of Acetic Acid in Vinegar by pptfiles

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									Percentage of Acetic Acid in Vinegar Background: In this experiment you will titrate a measured volume of Vinegar with a solution of NaOH of known concentration. The acid and the base react with one another in a 1:1 mole ratio. The Vinegar is placed in an Erlenmeyer flask, and phenolphthalein indicator is added. The NaOH solution is added from a burette into the flask containing the acid. During the first stages of the titration, the NaOH will be completely neutralized, and an excess of acid will remain. However, eventually there will be a point, the theoretical endpoint, at which the acid and the base have neutralized one another exactly, and no more base should be added to the flask. The phenolphthalein indicator is used to determine experimentally the endpoint, at which the base has neutralized the acid. Phenolphthalein is colourless in acid solution. It turns pink when the acid is completely neutralized and a slight excess of base is present. In this titration, a successful endpoint is achieved if one drop of base turns the solution in the flask from colourless to pink. In this experiment, we have separate burets for the acid and the base. It is advantageous to use two burets. If you should overshoot the endpoint by adding too much base, you will be able to add an additional measured volume of acid from the acid buret. The additional acid will neutralize the excess base, and you can then add more base to reach a new endpoint. Since you know both the concentration in moles per litre (mol/L) and the volume in mL (which you can convert to L) of the NaOH hydroxide, you can calculate the number of moles of base used. At the endpoint, the number of moles of acetic acid used equals the number of moles of NaOH used. Therefore, you know the number of moles of acetic acid in a measured volume of acid, and you can calculate the concentration of the acetic acid. For example, if 25.0 mL of 0.200 mol/L NaOH is able to neutralize 30.0 mL CH3COOH, we can calculate the concentration of the acid solution. First we determine the number of moles of NaOH: 0.200 mol/L = y mol / 0.0250 L y mol = (0.200 mol/L) x (0.0250L) = 0.005 mol In this case, moles of CH3COOH, equals moles of base at the endpoint. Therefore, 30.0 mL of CH3COOH, solution must contain 0.005 mol CH3COOH, and we can calculate the molar concentration of CH3COOH: x mol/L = (0.005 mol) / (0.0300 L) = 0.167 mol/L

Objective After completing this experiment you will be able to determine the acidity of a vinegar sample of unknown acidity using the titration technique Safety: Sodium hydroxide is corrosive. Avoid direct contact, but if any touches your hand, wash it off immediately. Safety glasses are mandatory Materials: Ring stand with buret clamp Erlenmeyer flask Buret Beakers (for discards) wash bottle White Vinegar Standardized solution of NaOH Phenolphthalein indicator Procedure 1. Prepare a table as shown. Record all your experimental data in the table. Record all measurements to the nearest 0.01 mL Titration of Acetic Acid with NaOH trials Trial #1 Concentration of NaOH Initial reading of acid buret Final reading of acid buret Volume of CH3COOH used Initial reading of base buret Final reading of base buret Volume of NaOH used _______mol/L ________ mL ________ mL ________ mL ________ mL ________ mL ________ mL Trial #2 ________mol/L ________ mL ________ mL ________ mL ________ mL ________ mL ________ mL Trial #3 _______mol/L ________ mL ________ mL ________ mL ________ mL ________ mL ________ mL

2. Using the burette transfer 10mL of vinegar into an Erlenmeyer flask. 3. Add some distilled water to the flask to increase the volume, thus making it easier to determine the colour change when the equivalence point is reached. 4. Add one or two drops of phenolphthalein solution to serve as an indicator

5. Titrate the vinegar with the standard solution of NaOH. Add the NaOH drop by drop near the end of the titration (when purple starts to appear). The equivalence point is most satisfactorily determined with the flask resting on a sheet of white paper and with a beaker of distilled water alongside for comparison purposes. 6. Record the final buret reading. 7. Clean Erlenmeyer flask and repeat for 2 more trials.

Calculations 1. Calculate the volumes of vinegar and NaOH used in each trial 2. Determine the moles of NaOH used in each of the three trials 3. Write the balanced equation for the reaction between vinegar and NaOH (vinegar is CH3COOH) 4. Use the mole ratio and results to determine the moles of vinegar used to neutralize the base in each trial 5. Given the moles and volumes of vinegar for each trial, calculate the molarities for the three trials. 6. Determine the mass of one mole of CH3COOH 7. IF your vinegar was 0.500 mol/L CH3COOH it would contain 0.500 moles of solute in one liter of solution. The mass of CH3COOH in 1 L of a 0.500 mol/L solution would be: mass CH3COOH 1 L vinegar = 0.500 moles CH3COOH x molar mass CH3COOH in g 1 L vinegar mol

8. If we assume that the density of vinegar is very close to 1.00 g/mL, the mass of 1.00L of vinegar is 1000g. The percent, by mass, of acetic acid can be expressed as: % CH3COOH = mass CH3COOH = mass CH3COOH x 100% 1.00L vinegar 1000g vinegar

Calculate the percentage of acetic acid in your vinegar sample. Questions: 1. If 27.31 mL of 0.2115 mol/L NaOH is able to neutralize 37.45 mL of CH3COOH what is the concentration of the acid? 2. Why does the pink colour, which forms at the point where the NaOH comes into contact with the solution in the flask, disappear more slowly near the endpoint? 3. Why is it a good idea to carry out titrations in triplicate? 4. Why might you be told to use distilled water to wash a drop of solution adhering to one of the buret tips into the Erlenmeyer flask?

5. Would the addition of several millilitres of distilled water to the Erlenmeyer flask during the titration affect the results of the titration? Explain your answer. 6. If 35.93 mL of 0.1590 M NaOH neutralizes 27.48 mL of sulphuric acid, what is the concentration of the sulphuric acid? Conclusions: Write Up: Be Sure to include the following: - Title - Name - Partners Name - Date - Objective - Materials (“refer to ....”) - Procedure (“refer to ...”) - Raw Data (this is your table) - A section for calculations - A section for questions - A section for conclusions. * Hints:    Be sure to label each section clearly Work neatly and methodically (don’t jump around) If you have any questions – ASK! Remember to answer the purpose and state at least 2 sources of error Compare your results to the %mass CH3COOH stated on the bottle


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