Lab 7 Instructor Notes

Lab 7: Titration of a Carbonated Beverage

  • Remind Students:
    • Goggles MUST be worn whenever ANY group has chemical at their station.
    • Review safety:
      • Acid is caustic.
      • Bases are caustic.
      • Hot glassware looks like cold glassware.
    • Make sure they secure all equipment in their bin when moving the bin between the cart and their stations.
    • Remind students to be CAREFUL with the burets.
    • All glassware must be cleaned and dry (in their bin) before leaving lab.
  • When reviewing the protocol explain:
    • Acids and bases.
    • What it means to be “triprotic.”
    • Part A:
      • Explain what it means that NaOH is hydroscopic.
      • Walk them through an example dilution problem.
      • Stress that it does not matter that they get exactly 3.2 mL NaOH as long as they get 3.0–4.0 mL, the experiment will work well.
      • Explain that they need to add the NaOH to the volumetric flask carefully so it does not spill. Add DI water so the meniscus sits on the line.
      • If they do not have a cork, explain how to use parafilm to cover the top and use their thumb to secure the parafilm when inverting.
      • They must swirl and invert the volumetric flask several times to ensure it is properly mixed. Stress that the inversion should not be done near the counter (do not shatter the flask).
    • Part B:
      • Explain why standardization is necessary.
      • Walk them through how to prepare the buret.
        • Explain that putting water through the buret makes sure there is no leak.
        • Students need to make sure the stopcock is closed (remind them closed = parallel to floor). They should also ensure it is securely connected (no gaps between glass and Teflon).
        • Use a funnel when adding any solution. This should also be done without lifting solutions above their head. They can lower the buret by moving the ring stand to the edge of the counter briefly. But safety MUST be considered here.
        • The buret will NOT work (drip) without at least 5-10 mL solution in there the first time, so you will need to assure them it is not really broken.
        • Once they have rinsed with water, they should rise with their prepared NaOH solution. They should NOT use the concentrated NaOH. Remind them that the purpose of this is to ensure the solution has the same molarity throughout.
        • Remind them that the amount of NaOH should be close to 0 but they should NOT waste time getting to 0.00.
        • Burets must be read to 2 decimal spaces. Do not change the numbers, record them exactly as they are (they often try to say 47.50 instead of 2.50).
        • It is helpful to draw a diagram of the flask on the board. What should be included? The g KHP, indicator and water should all be visible.
        • Remind them that the water does not change the mol KHP but makes it easier to see.
        • They should titrate until the ENTIRE solution remains a pale pink color.
        • There is only 1 drop difference between the pale pink and dark pink endpoint.
        • Show them the correct way to add drops of NaOH.
        • Remind them not to rest their hand on the stopcock (or it can fall out and they will have to start over).
        • The second trial should go faster because they can add ~1/2 the volume of the first trial fast, then go drop-wise.
        • Walk them through an example calculation for finding Molarity of NaOH from the titration.
      • Part C:
        • It is nice if you heat or have them heat the Sprite before beginning Part B.
        • The soda should not boil, only heat for a bit and then cool. Hot glassware looks like cold glassware. Be careful.
        • It is helpful to include a picture of the flask with all materials that belong here (water, sprite, and indicator).
        • The titration here is trickier. The endpoint must be visible in the entire solution for ~ 5–10 seconds.
        • An example calculation may be helpful.
      • Part D:
        • The titration of the solid citric acid is really helpful for them to see that this is a triprotic acid.
        • Draw a picture indicating everything that must be in the flask.
        • Walk them through this calculation as well.