Past Exam Questions

Question 1

Describe how the 25.0 cm cubed sample of the original bleach solution was diluted to exactly 500 cm cubed
(7)

Answer 1

Using a pipette filler, pipette 25 cm cubed of the concentrated bleach into a clean 500 cm cubed volumetric flask

Add deionised water until the water is near the mark

Add the water drop by drop using a dropper

Bring the bottom of the meniscus up to the mark on the volumetric flask

Make sure you read the bottom of the meniscus at eye level

Stopper the flask

Invert the flask several times to make sure the solution has the same concentration throughout

Question 2

What colour developed when he potassium iodide and the sulfuric acid reacted with the diluted bleach in the conical flask?

Answer 2

A reddish-brown colour is observed

Question 3

Give two reasons why excess potassium iodide was used.

Answer 3

To ensure that all the hypochlorite in the bleach had fully reacted (maximum amount of iodine produced)

To keep the iodine formed in solution

Question 4

What was the purpose of standing the conical flask on a white tile during the titration?

Answer 4

So that the colour change at the end point could be more easily seen

Question 5

Name the indicator used in the titrations

Answer 5

Starch

Question 6

State the colour change observed at the end point

Answer 6

Blue-black to colourless