Determination of the % of hydrochlorite in bleach

Question 1

Apparatus

Answer 1

Safety glasses, funnel, burette, pipette, conical flask, volumetric flask, retort stand, wash bottle with deionised water white tile.

Question 2

Materials

Answer 2

Standard solution of sodium thiosulphate, bleach, potassium iodide solution, dilute sulphuric acid.

Question 3

What is the first step in the method?

Answer 3

All glassware was rinsed with deionised water to clean them,

Question 4

What happens after the glassware was rinsed?

Answer 4

25 cm³ of original bleach were added to a volumetric flask and deionised water was added to make the solution up to 250cm³, observing the usual precautions.

Question 5

What happens after the original bleach was added to the volumetric flask and deionised water.

Answer 5

The pipette was rinsed with some of the diluted bleach solution and then 25cm³ of the bleach solution were added to a clean conical flask.

Question 6

What happens after the pipette was rinsed with some of the diluted bleach solution?

Answer 6

To this solution of diluted bleach about 20cm³ of dilute sulphuric acid were added along with about 10 cm³ of potassium iodide.

Question 7

What colour was the solution after about 20cm³ of dilute sulphuric acid were added along with 10cm³ of potassium iodide.

Answer 7

Red-brown colour

Question 8

Why was the solution red-brown colour after the dilute sulphuric acid and potassium iodide was added to the solution?

Answer 8

Due to the liberated iodine.

Question 9

What happens after the solution after about 20cm³ of dilute sulphuric acid were added along with 10cm³ of potassium iodide?

Answer 9

The burette was filled with the standard sodium thiosulphate solution, observing the usual precautions.

Question 10

What happens after the burette was filled with the standard sodium thiosulphate solution?

Answer 10

The solutions were titrated until a straw-yellow colour was reached and then some starch indicator was added with turns the solution in the conical flask a blue-black colour.

Question 11

What indicator was used?

Answer 11

Starch

Question 12

When did the titration end?

Answer 12

When the colour changed from blue-black to colourless.

Question 13

What colour was the solution titrated until? (before starch indicator was added)

Answer 13

Straw-yellow colour

Question 14

What is the ‘last step’?

Answer 14

Three titrations were carried out.

Question 15

What were the results from the burette?

Answer 15

Rough titration - 28.7cm³
2nd titration - 28.5cm³
3rd titration - 28.6cm³

Question 16

What is the bleach (hypochlorite ion) in the titration?

Answer 16

An oxidising agent

Question 17

What does the bleach (hypochlorite ion) do?

Answer 17

It liberates iodine from solutions of excess potassium iodide in a 1:1 ratio.

Question 18

How can the amount of bleach (hypochlorite ions) be calculated?

Answer 18

By reacting the iodine that was liberated with a previously standardised solution of sodium thiosulphate.

Question 19

What is the oxidising agent in this reaction?

Answer 19

Bleach

Question 20

What is the limiting reactant in this reaction?

Answer 20

Bleach

Question 21

Why was the bleach diluted?

Answer 21

The bleach was too concentrated and would require an excessive of sodium thiosulfate to react with the iodine that the bleach liberated.

Question 22

Describe how the 25.0cm³ sample of the original bleach solution was diluted to exactly 500cm³. (L.C)

Answer 22

• Pipetted into 500cm³ volumetric flask
• Add deionised water until near mark
• Add dropwise
• Stopper and invert several times and mix thoroughly

Question 23

What colour developed when the potassium iodide and the sulfuric acid reacted with the diluted bleach in the conical flask? (L.C)

Answer 23

Brown / red

Question 24

Give two reasons why excess potassium iodide was used. (L.C)

Answer 24

• So that all the bleach has reacted

- To keep the iodine in solution

Question 25

What was the purpose of standing the conical flask on a white tile during the titration? (L.C)

Answer 25

So that colour-change (end point) is clearer

Question 26

Name the indicator used in the titrations and state the colour change observed at the end point. (L.C)

Answer 26

Starch

Blue-black to colourless

Question 27

Why was the bleach diluted before carrying out the titrations? (L.C)

Answer 27

The bleach was too concentrated and would require an excessive of sodium thiosulfate to react with the iodine that the bleach liberated.

Question 28

At what stage in the titration is the indicator added?(L.C)

Answer 28

When the solutions were titrated until a straw-yellow colour was reached.

Question 29

What colour change indicates the end point? (L.C)

Answer 29

Blue-black to colourless

Question 30

Name the indicator used. (L.C)

Answer 30

Starch

Question 31

Why is excess potassium iodide used? (L.C)

Answer 31

• So that all the bleach has reacted

- To keep the iodine in solution

Question 32

Identify the piece of apparatus that should be used to transfer the bleach to the volumetric flask. (L.C)

Answer 32

Pipette