Q1 - Titrations

Question 1

State two properties of hydrated ammonium iron (II) sulfate that make it suitable for use as a primary standard

Answer 1

• pure
• solid
• soluble
• does not absorb water
• not easily oxidised

Question 2

Explain the term standardised

Answer 2

The concentration (molarity) is found by means of a titration

Question 3

Explain the term primary standard

Answer 3

Can be used to make up a solution of known concentration

It is pure, stable and anhydrous

Question 4

State one precaution that should be taken as the end point of a titration is approached

Explain now this contributes to the accuracy of the result

Answer 4

Add drop by drop → accuracy

Swirl flask and wash down sides with deionised water → all solution is thoroughly mixed

Question 5

Describe how the level of liquid in the burette is adjusted to the zero mark

Answer 5

Fill to above the mark and then let it flow into a waste beaker until the bottom of the meniscus is at zero when read from eye level

Question 6

Describe in detail the procedures involved in measuring a 25ml sample of the vinegar and diluting it using deionised water to exactly 250ml

Answer 6

• rinse a clean pipette with deionised water, and then vinegar
• rinse a volumetric flask with deionised water
• using a pipette filler, pipette 25ml of vinegar into the volumetric flask
• fill the flask with deionised water until the bottom of the meniscus is at the graduation mark when read from eye level
• insert stopper and invert 20 times

Question 7

Ethanoic acid in vinegar:
Why was the vinegar diluted?

Answer 7

Vinegar is too concentrated and would require a large volume of sodium hydroxide

Question 8

To determine the concentration of ethanoic acid in vinegar:
Name a suitable indicator
Why suitable?
Colour change at endpoint

Answer 8

Phenolphthalein

Weak acid / strong base

Pink → colourless

Question 9

To determine the amount of water of crystallisation in hydrated sodium carbonate:
Identify a primary standard reagent which could have been used to standardise the HCl

Answer 9

Anhydrous sodium carbonate

Question 10

To determine the amount of water of crystallisation in hydrated sodium carbonate:
Name a suitable indicator for the titration and state the colour change

Answer 10

Methyl orange (strong acid / weak base)

Yellow/orange → pink

Question 11

Explain why only a small amount of indicator should be used

Answer 11

Indicators are weak acids or bases and therefore will affect the titration point

Question 12

Describe the procedure for rinsing the burette before filling it with the solution it is to deliver

Answer 12

Rinse with deionised water and the solution it will contain

Question 13

Why is it important to fill below the tap of the burette?

Answer 13

This is part of the 50cm3 and it will contain air if not filled. Therefore giving you an inaccurate titration figure.

Question 14

Why are iron tablets sometimes medically prescribed?

Answer 14

Prevent anaemia

Question 15

Why must potassium permanganate solutions be standardised?

Answer 15

Potassium permanganate is not a primary standard

Question 16

To make up a solution with a solid solute

Answer 16

• rinse from clock glass into a beaker containing deionised water
• stir to dissolve
• add to a volumetric flask using a funnel
• rinse contents of beaker into flask using a wash bottle
• add deionised water to volumetric flask until the bottom of the meniscus is on the mark when read from eye level
• insert stopper and invert several times

Question 17

How to dilute a solution of bleach or vinegar

Answer 17

• use pipette filler to fill pipette
• allow to drain into volumetric flask under gravity
• add deionised water until the bottom of the meniscus is at the graduation mark when read from eye level
• Insert stopper and invert several times

Question 18

Procedure for preparing burette

Answer 18

• rinse with deionised water
• rinse with solution it will contain
• clamp vertically in retort stand
• add reagent to above 0 mark using funnel
• open the tap to fill below tap
• close tap when bottom of meniscus is on the mark when read from eye level

Question 19

What is the importance of filling the part below the tap in the burette?

Answer 19

• Causes inaccurate reading
• Air will be displaced during titration
• Affect result
• Some of the measured volume replaces air

Question 20

Procedure for preparing pipette

Answer 20

• rinse with deionised water
• rinse with solution it will contain
• fill using a pipette filler
• adjust to have the bottom of the meniscus on the mark when read from eye level
• drain under gravity into flask
• knock against side of flask to remove final drop

Question 21

Why is the conical flask placed on a white tile?

Answer 21

So the colour change at the end point can be seen clearly

Question 22

Why is potassium permanganate a secondary standard?

Answer 22

It must be standardised immediately before use as it decomposes in strong sunlight

Question 23

Why must potassium permanganate solutions be standardised?

Answer 23

Potassium permanganate is not a primary standard

Question 24

Explain why distilled water instead of deionised water would provide a more accurate result

Answer 24

deionised water is not as pure because it only has ions removed and may still contain dissolved substances

Question 25

To determine the percentage (w/v) of hypochlorite in bleach:
Why was excess potassium iodide used?

Answer 25

• keep iodine in solution as soluble potassium triiodide
• ensure all hypochlorite in bleach has recaptured

Question 26

To determine the percentage (w/v) of hypochlorite in bleach:
What colour developed when the potassium iodide and the sulfuric acid reacted with the diluted bleach in the conical flask?

Answer 26

Reddish-brown coloun

Question 27

Why is it not recommended procedure to pipette directly from the volumetric flask?

Answer 27

Any impurities on the pipette would contaminate the entire solution in the volumetric flask

Question 28

Why is it important not to add too much indicator

Answer 28

Indicators are either weak acids or weak bases and adding too much could affect the accuracy of the result
Also, colour could be too intense, making it difficult to see the colour change at end paint

Question 29

Ethanoic acid in vinegar:
Why is it important to use colourless vinegar rather than brown vinegar

Answer 29

The brown colour of vinegar would mask the colour change at the end point

Question 30

Ethanoic acid in vinegar:
Why is the vinegar diluted?

Answer 30

To ensure the end point is within the limits of the burette
↳ if undiluted vinegar was used a very large amount of sodium hydroxide solution would be required to neutralise it

Question 31

Ethanoic acid in vinegar:
What colour change is observed?

Answer 31

Phenolphthalein

Pink → colourless

Question 32

What precautions should you take when setting up the burette for titration?

Answer 32

• clamp burette vertically using retort stand
• rinse burette with deionised water and the solution it is going to contain
• fill burette with solution using a funnel, open the tap to ensure the space below the tap is filled

Question 33

Ethanoic acid in vinegar:
Outline the correct procedure for diluting the vinegar

Answer 33

• pipette vinegar into volumetric flask
• allow pipette to drain under gravity
• add deionised water until the bottom of the meniscus is at the graduation mark
• invert flask several times

Question 34

Ethanoic acid in vinegar:
What is meant by %w/v ?

Answer 34

Percentage weight per volume