Mandatory Student Experiment No. 11

Question 1

Explain how iodine, a non-polar substance of very low water solubility, is brought into aqueous solution

Answer 1

Reacts with iodide

Question 2

The iodine solution was made up in a 500cm^3 volumetric flask. Describe the procedure for measuring 25.0cm^3 of this solution into a conical flask

Answer 2

Pour iodine solution into a clean, dry beaker
Rinse a pipette with deionised water and iodine solution
Fill using a pipette filler until at the bottom of meniscus when read from eye level
Tap pipette against wall of conical flask to let the last drip out

Question 3

Name a suitable indicator for this experiment

Answer 3

Starch

Question 4

At what stage is the indicator added

Answer 4

When solution in conical flask is light yellow

Question 5

State the colour change at the end point in the presence of the indicator

Answer 5

Blue-black to colourless

Question 6

Explain why the use of distilled water instead of deionised water throughout the experiment would be likely to ensure a more accurate result

Answer 6

As deionised water could contain non-ionic substances that could be oxidised or reduced such as chlorine which is an oxidising agent

Question 7

Explain fully what a primary standard is

Answer 7

A substance that can be obtained in a stable, pure and soluble solid form so that it can be weighed out and dissolved in water to give a solution of accurately known concentration

Question 8

What must be added to bring iodine into aqueous solution?

Answer 8

Iodide

Question 9

What sequence of colours was observed in the conical flask from the start of the titration until the end point was reached?

Answer 9

Brown
Light yellow
Blue-black
Colourless