BLEACH

Question 1

Describe how the 25cm^3 sample of the original bleach solution was diluted to exactly 500cm^3

Answer 1

pipette into 500 cm3 volumetric flask
add deionised water until near calibration mark and then add dropwise
until bottom of meniscus up to mark at eye-level
stopper and invert several times to ensure solution is homogeneous

Question 2

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

Answer 2

Brown/red

Question 3

Give two reasons why excess potassium iodide was used

Answer 3

• > so that all bleach [hypochlorite] has reacted

- > to keep iodine in solution

Question 4

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

Answer 4

Colour change is clearer

Question 5

Name the indicator used in the titration and state the colour change observed at the end point

Answer 5

Starch

Blue/black->colourless

Question 6

Why was it necessary to dilute the bleach?

Answer 6

Household bleach which is used is too concentrated. If the bleach were not diluted, excessive amounts of potassium iodide and sodium thiosulfate would be needed in the experiment to get a reasonable titration figure

Question 7

Why is it particularly important in the experiment to use a pipette filler when placing the household bleach in the volumetric flask

Answer 7

Since bleach is a harmful substance, it should never be pipetted by mouth in case some of it is accidently swallowed

Question 8

Why was dilute sulfuric acid added to the bleach solution in the conical flask

Answer 8

To supply H+ ions and ensure the reduction of ClO- to Cl- occurs

Question 9

Explain why hydrochloric acid should not be used when acidifying the bleach

Answer 9

Chlorine gas could be produced

Question 10

What colour is observed in the conical flask after the addition of the dilute sulfuric acid and the potassium iodide solution to the bleach solution

Answer 10

Reddish/Brown colour - iodine liberated

Question 11

Why is the indicator not added until the solution in the conical flask becomes a pale yellow

Answer 11

If the starch is added at an early stage of the titration, the iodine present may become strongly absorbed onto the starch and this makes the titrations less accurate. In addition, when the pale yellow colour is observed in the conical flask, it tells us that the end point is quite near. Therefore, when the starch is added at this stage, adding the thiosulfate drop by drop from this stage onwards helps to obtain an accurate titration figure

Question 12

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

Answer 12

Red-Brown -> I2 liberated in conical flask
Straw Yellow -> addition of Na2S2O3
Blue-Black -> addition of starch indicator to form a starch-iodine complex
Colourless -> at end point

Question 13

Outline three operations involving the conical flask and its contents during the titration

Answer 13

Swirling action - ensure homogenous solution
Wash down sides of flask with deionised water - ensure everything is being reacted
Flask on white tile (See colour change clearer)

Question 14

What is in the burette and pipette?

Answer 14

Burette - Sodium thiosulfate solution

Pipette - Diluted bleach solution

Question 15

What is in the conical flask during the titration

Answer 15

Diluted bleach + dilute sulfuric acid + potassium iodide

Question 16

Why could you not use hydrochloric acid when acidifying the bleach

Answer 16

HCL would react with hypochlorite to liberate chlorine gas

Question 17

Why are some bleaches such as domestos not suitable?

Answer 17

When diluted it forms suds which makes it difficult to accurately make up the diluted solution