making a report for Experiment 2 Acetic Acid Content of Vinegar
The goal of this experiment is to determine accurately theconcentration of acetic acid in vinegar via volumetric analysis, making use of the reaction of acetic acid with a strong base, sodium hydroxide. Part A Standardisation of 0.1 M sodium hydroxide with the primary standardpotassium hydrogenphthalate.
(A1) In a clean, dry 100 mL beaker take about 40 mL of the approximately 1 M solution of NaOH. Dilute the solution by a factor of ten by pipetting 25.00 mL of it into a 250 mL volumetric flask and making the volume up to 250.0 mL with deionised water. Swirl/shake/invert for about 2 minutes to mix the solution thoroughly. (A2) Transfer this solution to a 250 mL conical flask and label the solution as 0.1 M NaOH. Retain this as your stock of 0.1 M sodium hydroxide for standardization. (A3) Use an analytical balance in the balance room at the back of the lab to weigh out accurately about 4 g (anywhere in the range 3.9 – 4.1 g will do) of potassium hydrogenphthalate from the sample tubes provided into a clean, dry 100 mL beaker (see Skill 3.2). You need to know the mass accurately to ±0.0002 g for your subsequent calculations. Record the exact mass in your logbook. (A4) Back at your bench, add about 80 mL of deionised water to the potassium hydrogenphthalate solid and stir to dissolve it. Transfer the solution to a 50 mL volumetric flask with the aid of the stirring rod and a plastic funnel. Add some more deionised water to the flask until just below the mark, mix to homogenise the solution and then make the solution up to the mark with deionised water, adding the water with a dropper. (Note: Never add water directly from a wash bottle. You can easily go past the mark and you’d have to throw the solution away and start again). Mix. Transfer the solution to a 250 mL conical flask and label it as KC8H5O4. For your logbook: Calculate the molar concentration of the potassium hydrogenphthalate, KC8H5O4, solution. (A5) Read Skill 4.8. Use a pipette to transfer 25.00 mL of your 0.1 M NaOH solution to a conical flask. Add 2 drops of phenolphthalein indicator and swirl to mix. Carry out a rapid titration with your KC8H5O4 solution in the burette, gently swirling the conical flask as you titrate. In your logbook record the rough volume required to reach the endpoint (i.e. the volume required to just observe a persistent colour change). (A6) Pipette another 25.00 mL of your 0.1 M NaOH into a clean conical flask. Fill the burette again with the KC8H5O4 solution and record the initial volume in the burette (in mL to two decimal places) in your logbook. Then titrate the KC8H5O4solution from the burette into the NaOH solution, slowing down to drop by drop and then split drops about 2 mL before the rough titration volume from your last titration. Record the final volume of KC8H5O4 in the burette at the endpoint and hence calculate the total volume of KC8H5O4 solution added (the titre value) in your logbook. (A7) Repeat accurate titrations (as described in (A6)) until you have 3 concordant titre values that are within 0.10 mL of one another. For your logbook: Calculate the exact concentration of your roughly 0.1 M solution of NaOH.
Check your concentration with your demonstrator before proceeding to Part B.
Part B: Titration of vinegar with standard sodium hydroxide (B1) Dilute one of the vinegars by a factor of ten by pipetting 25.00 mL of it into a 250.0 mL volumetric flask and making it up to the mark with deionised water. Label the solution, e.g. dilute vinegar or dil. CH3COOH (just as long as you know what it is). Record in your logbook which particular type of vinegar you used. (B2) Perform a series of titrations with 25.00 mL of your standardized 0.1 M NaOH solution in the conical flask and the diluted vinegar in the burette (thoroughly rinsed from the last series of titrations). Don’t forget to add 2 drops of indicator to your NaOH solution. As in Part A, the solution should start pink and your endpoint will be when the solution in the conical flask is persistently colourless on swirling. Repeat your titrations until you have 3 oncordant titre values within 0.10 mL of one another. Record your titre values in your logbook.