Titration - standardisation of potassium manganate using ammionium iron sulphate crystalline solution.

Question 1

What is the equation for ammonium iron (II) sulphate?

Answer 1

(NH₄)₂SO₄FeSO₄.6H₂0

Question 2

Is ammonium iron (II) sulphate a primary or a secondary standard?

Answer 2

Primary

Question 3

Why is it necessary that ammonium iron (II) sulphate is a primary standard?

Answer 3

So it can be used to standardise potassium manganate (VII).

Question 4

What is first done to the ammonium iron (II) sulphate?

Answer 4

Pipetted into the conical flask

Question 5

Is Fe(+2) easily oxidised to Fe(+3)?

Answer 5

Yes

Question 6

What is Fe(+2) oxidised to?

Answer 6

Fe(+3)

Question 7

How is Fe(+2) oxidised to Fe(+3)?

Answer 7

By atmospheric oxygen

Question 8

Why is ammonium iron (II) sulphate used?

Answer 8

To prevent the Fe(+2) from being oxidised by atmospheric oxygen to Fe(+3).

Question 9

Why can ammonium iron (II) sulphate prevent Fe(+2) from being oxidised by atmospheric oxygen to Fe(+3)?

Answer 9

As it is quite stable in air.

Question 10

Does ammonium iron (II) sulphate have a high or low molecular mass?

Answer 10

High

Question 11

What does ammonium iron (II) sulphate’s high molecular mass ensure?

Answer 11

Ensures accuracy when weighing.

Question 12

What is added to the volumetric flask when making up ammonium iron (II) sulphate?

Answer 12

Sulphuric acid.

Question 13

Why is sulphuric acid added to the volumetric flask when making up the ammonium iron (II) sulphate?

Answer 13

To prevent aerial oxidation of Fe(+2) to Fe(+3).

Question 14

What is reduced during the titration?

Answer 14

Potassium manganate (VII)

Question 15

What is the equation for potassium manganate (VII)?

Answer 15

KMnO₄

Question 16

What is the oxidising agent?

Answer 16

Potassium manganate (VII)

Question 17

Is potassium manganate a primary or secondary standard?

Answer 17

Secondary

Question 18

Why is potassium manganate (VII) a secondary standard?

Answer 18

As it decomposes in strong sunlight and is also easily reduced.

Question 19

Does potassium manganate (VII) dissolve easily in water?

Answer 19

Yes

Question 20

What colour is potassium manganate (VII)?

Answer 20

Deep purple

Question 21

Why is potassium manganate (VII) deep purple?

Answer 21

Due to Mn(+7)

Question 22

Where is the potassium manganate (VII) placed?

Answer 22

Always placed in the burette.

Question 23

What conditions are titrations always carried out in?

Answer 23

Acidic conditions.

Question 24

Why are titrations always carried out in acidic conditions?

Answer 24

To ensure the complete reduction of Mn(+7) to Mn(+2). The sulfuric acid is added to the conical flask to ensure this.

Question 25

What does the formation of Mn(+2) act as in the reaction?

Answer 25

Autocatalyst

Question 26

What is the colour change in the reaction?

Answer 26

Purple to colourless

Question 27

What colour is Mn(+2)?

Answer 27

Colourless

Question 28

What colour is Mn(+7)?

Answer 28

Purple

Question 29

What happens if insufficient acid (H₂SO₄) is added to the conical flask?

Answer 29

The Mn(+7) is incompletely reduced to Mn(+4), which appears in the conical flask as a brown ppt of MnO₂.

Question 30

What is done to rectify if insufficient acid (H₂SO₄) is added?

Answer 30

Add more sulphuric acid to the conical flask.

Question 31

Can HCl or HNO₃ be used to acidify the solution in the conical flask?

Answer 31

No

Question 32

Why can't HNO₃ be used to acidify the solution in the conical flask?

Answer 32

As nitric acid is a powerful oxidising agent itself and will take part in the reaction.

Question 33

Why can't HCl be used to acidify the solution in the conical flask?

Answer 33

As the hydrochloric acid will be oxidised by the manganate (VII) solution.

Question 34

What is the first step in the titration?

Answer 34

The potassium manganate (VII) is placed into the burette observing the usual precautions.

Question 35

What happens after the potassium manganate placed into the burette?

Answer 35

The acidified ammonium iron (II) sulphate solution is pipetted into the conical flask observing the usual safety precautions.

Question 36

What happens after the acidified ammonium iron (II) sulphate solution is pipetted into the conical flask?

Answer 36

Dilute sulphuric acid is added to the conical flask.

Question 37

What happens when the purple manganate (VII) is added to the conical flask from the burette?

Answer 37

The colour change that occurs is purple to colourless.

Question 38

Why is the acidified ammonium iron (II) sulphate solution pipetted into the conical flask?

Answer 38

To prevent aerial oxidation of Fe(+2) to Fe(+3).

Question 39

Why is the dilute sulphuric acid added to the conical flask?

Answer 39

To ensure the complete reduction of Mn(+7) to Mn(+2).

Question 40

Does the colour change happen immediately?

Answer 40

Yes

Question 41

What does the fact that the colour change is immediate show?

Answer 41

That the reaction is very fast at room temperature.

Question 42

When is the end-point indicated?

Answer 42

When the colourless solution in the conical flask retains a permanent pink tinge.

Question 43

Why does the permanent pink tinge represent the end-point?

Answer 43

As there are no more Fe(+2) present to reduce the purple Mn(+7) to colourless Mn(+2) as it has all been oxidised to Fe(+3).

Question 44

Is there an indicator needed?

Answer 44

No

Question 45

Why is there no indicator needed?

Answer 45

As the manganate (VII) acts as its own indicator due to the colour changes that occur during and at the end-point of the reaction.

Question 46

What is the reaction ratio between MnO₄- and Fe₂+?

Answer 46

5:1

Question 47

What happens when Fe(+2) is oxidised to Fe(+3)?

Answer 47

It loses 1e

Question 48

What is another variation of this experiment?

Answer 48

To ask about the purity of the crystals.

Question 49

What is the balanced ionic equation?

Answer 49

MnO₄- + 5Fe₂+ + 8H+ → Mn₂+ + 5Fe₃+ + 4H₂0

Question 50

What happens when Mn(+7) is reduced to Mn(+2)?

Answer 50

Mn(+7) loses 5e