Question

1. Graduated cylinders are not as precisely calibrated as are burets or volumetric pipets. Briefly explain why it is acceptable to measure the KI and HCI solutions used in the titration with graduated cylinders rather than with pipets or burets. 2. Would the following procedural errors result in an incorrectly high or low calculated precent NaOCI in commercial bleaching solution? Briefly explain. (1) A student failed to allow the volumetric pipet to drain completely when transferring the diluted bleaching solution to the Erlenmeyer flask. (2) A student blew the last drops of solution from the pipet into the volumetric flask when transfeering commercial bleaching solution to the flask. (3) A student began a titration with an air bubble in the buret tip. Thhe bubbl came out of the tip after 5 mL of Na2S2O3 solution had been released.

Answer 1

1) Graduated cylinders are not as precisely calibrated as are burets or volumetric pipets.It is acceptable to measure the KI and HCl solutions used in the titration with graduated cylinders rather than with pipettes or burettes because we are not after a specific amount, we are after excess.

2)

(1) If a student failed to drain completely the volumetric pipette when transferring the diluted bleach, this error would result in an incorrectly low percent of NaOCl. NaOCl is the active ingredient in bleach. If incorrect low amounts are transferred to the Erlenmeyer flask, then the NaOCl concentration is also incorrect.

(2) If a student below drops of commercial bleaching solution into the volumetric flask, then that student has introduced another set of chemicals from the moisture of their mouth into the solution, altering the experiment and altering the results.

(3) Having a bubble in the burette tip containing 5 mL of Na₂SO₃ results in an incorrectly high calculated percent NaOCl in the commercial bleaching solution. Na₂SO₃ is used to determine the amount of I₂ formed by titration. It reconverts I₂ made from the reaction of OCl^- ion and I^- ion into I- ions again. So, transferring less Na₂SO3 to the pipette affects the concentration of OCl^- ion in the bleaching solution.