Experiment 20: Analysis of Vinegar Materials: graduated cylinder 6 M NaOH: Dilute Sodium Hydroxide 1000 ml Florence Flask & stopper KHC 8 H 4 O 4 : Potassium Hydrogen Phthalate (KHP) 125 ml Erlenmeyer Flasks (3) Phenolphthalein Indicator Buret Stand (or ring stand) Unknown Vinegar Solution (3.00-5.00 % HC 2 H 3 O 2 ) Buret Clamp (or utility clamp) Funnel, small plastic (optional) 50 ml Buret 10 ml Pipet and Pipet Bulb 100 ml Beaker 150 ml Beaker Wash Bottle with Deionized Water Preparation of Standard Sodium Hydroxide Solution Procedure A. 1. Preparing the NaOH solution in Florence Flask Half fill a 1000 ml Florence Flask with ~500 ml of Distilled Water. Measure ~20 ml of 6 M NaOH into a graduated cylinder and pour into the flask. Stopper the flask and carefully swirl to mix solution.
Procedure A. 2. Conditioning a Buret Set up a buret. Use small funnel and half fill the buret with NaOH solution from the Florence flask. Allow some solution to flow out the tip; then, invert and drain the buret. Check that there are no air bubbles in the tip of the buret. Make sure there are no bubbles, like these, in the tip of the buret. This tip looks good: no bubbles! Also, make sure the buret does not leak when turned off.
Procedure A. 3. Carefully fill the buret; do not overfill the buret. You ll record an initial reading and a final reading, then take the difference; so, it does not matter if it is at zero, exactly; but, in this case, fill and adjust till the buret is exactly at 0.00 ml. 0.00 ml 22.90 ml 22.25 ml 0.00 ml
Procedure A. 4. Label three 125 ml Erlenmeyer flasks #1, #2, and #3. Accurately weigh out ~1 g of KHP into each flask. Add ~25 ml of DI water to each flask. Heat if necessary to dissolve the KHP crystals. Fold the weighing paper in half then open it to have a crease running across. Gently place weighing paper on the analytical balance Push the front bar down gently to tare, or zero the mass. Use a Scoopula to transfer about 1 gram onto the weighing paper. Carefully remove the paper and KHP. A crease in the paper made this easier
Procedure A. 5. Titration Buret Stand, Buret Clamp,Buret, and 250 ml Erlenmeyer Flask with sample, and 1 drop of phenolphthalein. Drain some NaOH through tip to clear bubbles. Position Erlenmeyer flask #1 under the buret. Record initial buret reading (± 0.05 ml) Add a drop of phenolphthalein indicator to the flask. titrate the KHP sample to a permanent pink endpoint Record the final buret reading (± 0.05 ml) Your instructor will explain what to watch for during the titration; like the pink color where the drops hit the solution, and the persistence of that color. Stop the titration when the solution just turns pink, and stays pink.
Procedure A. 6. Refill the buret and repeat, record for flask #2 Procedure A. 7. Refill the buret and repeat, record for flask #3. Procedure A. 8. Calculate the molarity of the NaOH solution for each trial; record the average molarity of NaOH in the Data table of Procedure B. NOTE: SAVE THE NaOH in the FLORENCE FLASK for PROCEDURE B. Procedure B. 1. Obtain ~50 ml of Vinegar solution in a dry 100 ml beaker. Record the unknown number in the Data Table. Procedure B. 2. Condition the pipet with unknown vinegar solution, and transfer a 10.0 ml sample into each 125 ml flask. Add ~25 ml of distilled water into each flask. It is not necessary to use dry flasks. Procedure B. 3. Fill the buret with NaOH solution; adjust meniscus to zero, record initial buret reading as 0.00 ml. Add a drop of phenolphthalein to flask #1. Titrate the vinegar sample to a pink endpoint. Record the final buret reading. Procedure B. 4. Refill the buret to 0.00 ml, add one drop of phenolphthalein to flask #2 and titrate the sample to a pink endpoint. Procedure B. 5. Refill, repeat for flask #3. NOTE: By adjusting the meniscus in the buret to 0.00 ml, the endpoints for flasks #1, #2, and #3 should be the same. Procedure B. 6. Calculate the molarity of acetic acid, HC 2 H 3 O 2 in the vinegar solution. Procedure B. 7. Convert the molarity of HC 2 H 3 O 2 (60.06 g/mol) to mass/mass percent concentration. Assume the density is 1.01 g/ml for the unknown vinegar solution. NOTE: When you re finished with this lab, please rinse the buret and all glassware with distilled water to remove traces of NaOH solution.