RQP 1: Make up a volumetric solution and carry out a simple acid-base titration

Question 1

Describe how to make a standard solution with a known mass of solid

Answer 1

• weigh the weighing boat containing the required mass of solid on a 2 d.p balance
• tranfer to a beaker
• reweigh weighing boat
• record the difference in mass
• Add 100cm^3 of distilled water to the beaker.
• Use a glass rod to stir to help dissolve the solid.
• pour the solution into a 250cm^3 volumetric flask via a funnel
• rinse beaker and funnel with distilled water and add washing from the beaker and glass rod to the volumetric flask.
• make up to the mark with distilled water using a dropping pipette for the last drops.
• invert flask several times to ensure uniform solution.

Question 2

What step should be taken if the solid being dissolved to make the standard solution doesnt dissolve in cold water

Answer 2

Heat the beaker and contents gently until all of the solid is dissolved

Question 3

What problem may arise when making a standard solution of dark liquids such as potassium manganate

Answer 3

It may be difficult to see the miniscus to make sure that the bottom of it touches the mark

Question 4

Why should you not heat or put hot solutions in a graduated flask

Answer 4

the heat would cause the flask to expand and then the volume would be incorrect- it would not reflect the mark on the volumetric flask

Question 5

What is the method for measuring mass accurately

Answer 5

1. measure mass on a 2 or 3 d.p balance of a weighing boat/bottle with the required quantity of solid in it
2. empty mass into reaction vessel/flask
3. reweigh the empty weghing bottle/boat
4. subtract the mass of the empty weighing bottle/boat from the first reading to give the exact mass actually added.

Question 6

Describe how to dilute a 25cm^3 solution (to make 250cm^3 volume)

Answer 6

• pipette 25 cm^3 of the original solution into a 250 cm^3 volumetric flask
• make up to the mark with distilled water using a dropping pipette for the last few drops
• invert the flask several times to ensure uniform solution

Question 7

What is more accurate- a volumetric pipette or a measuring cylinder and why?

Answer 7

A volumetric pippete has a smaller uncertainty than a measuring cylinder

Question 8

What should you use to make up to the mark in a volumetric flask

Answer 8

A teat pipette which ensures that the volume of solution is accurately measured and does not go over the line.

Question 9

In terms of equipment, what does ‘titrate solution A with solution B’ mean?

Answer 9

Solution A is in the conical flask and solution B is in the burette

Question 10

Describe the general method for carrying out an acid-base titration

Answer 10

1. Rinse the equipment (burette with acid, pipette with alkali, conical flask with distilled water)
2. Pipette 25 cm^3 of alkali into conical flask.
3. Touch surface of alkali with pipette (to enmsure correct amount is added).
4. adds acid solution from burette
5. make sure the jet space in the burette is filled with acid (no air bubbles)
6. add a few drops of indicator and refer to colour change at the end point for that indicator.
7. use a white tile underneath the flask to help observe the colour change
8. add acid to the alkali whilst swirling the mixture and add dropwise at the end point
9. note burette reading before and after each addition of acid
10. repeat titration until at least 2 concordant results are obtained.

Question 11

What are concordant titres

Answer 11

Titres within 0.1 of each other

Question 12

What is the colour change observed when using phenolphthalein as the indicator and when should this indicator be used

Answer 12

• colour change is pink (alkali) to colourless (acid)
• At the end point the pink colour disappears
• use phenolphthalein if NaOH is used

Question 13

What is the colour change observed when using methyl orange as the indicator and when should this indicator be used

Answer 13

• colour change is yellow (alkali) to red (acid)
• at the end point it is orange
• use methyl orange if HCl is used

Question 14

Why is a conical flask used in preference to a beaker when doing a titration

Answer 14

It is easier to swirl the mixture in a conical flaks without spilling the contents

Question 15

Describe in detail how to accurately use a pippette during a simple titration

Answer 15

• rise the pipette with the substance to go in it (often the alkali)
• pipette 25cm^3 of the solution into a concical flask
• A volumetric pipette will have a mark on its neck to show the level you fill to and the bottom of the miniscus should sit on this line.
• Touch the surface of the solution with the pipette to ensure the correct amout is added.

Question 16

Why must the burette be rinsed with the solution that is being put it in and what affect would not doing this have on the titration

Answer 16

• If it is not rinsed the acid or alkali added may be diluted by residual water in the burette or may react with substances left in the burette
• this would lead to the concentration of the substance being lowered and a larger titre beingng delivered.

Question 17

Why should you remove the funnel from the burette and what affect would not doing this have on the titration

Answer 17

• If the funnel was left on the burette, small drops of liquid may fall from the funnel during the titration and this woud lead to a false burette reading.
• The burette would give a lower titre volume.

Question 18

What must you do to the jet space of the burette before beginning a titration and what affect would not doing this have on the titration

Answer 18

• make sure the jet space in the burette is filled with the solution and any air bubbles are removed.
• if the jet space is not filled properly, it will lead to errors when it fills during the titration: the titre reading will be larger than expected

Question 19

How many decimal places should burette readings always be given to

Answer 19

2 d.p

Question 20

Whymust you only use a few drops of indicator in a titration

Answer 20

Indicators are generally weak acids so if too much is added they will affect the titration result.

Question 21

Over what pH range does phenolpthalein change, what is the colour change and what type of titration is it useful for

Answer 21

• phenolphalein changes between pH 8.2-10
• it changes from colourless at lower pHs to pink at higher pHs
• it is useful for weak acid/strong base titrations

Question 22

Over what pH range does methyl orange change colour, what is its colour change and what type of titration is it useful for

Answer 22

• methyl orange changes colour between pH 3-4.5
• it is red at low pHs and orange/yellow at high pHs
• it is useful for strong acid-strong base and strong acid-weak base titrations

Question 23

If using an indicator during a titration, what extra piece of equipment should you use

Answer 23

You should place the concial flask on a white tile to help observe the colour change

Question 24

What should you do to ensure that you do not miss the end point of a titration

Answer 24

Add the solution from the burette drop wise when approaching the endpoint

Question 25

How many times should you repeat a titration

Answer 25

Until at least two concordant results (readings within 0.1 of each other) are obtained

Question 26

How do you make sure all of the acid/alkali on the side of thr concial flask is washed into the reaction mixture during a titration

Answer 26

• During a titration distilled water can be added to the conical flask to wash the sides of the flask and ensure that all of the acid/alkali is reacting
• it does not affect the titration reading as water does not react with the reagents or change the number of moles of acid added.

Question 27

How should you wash the conical flask between titrations

Answer 27

Only distilled water should be used to wash out conical flasks between titrations because it does not add any extra moles of reagents

Question 28

What are the hazards associated with simple acid-base titrations

Answer 28

• acids and alkalis are corrosive
• at low concentraions acids are irritants

Question 29

What safety precuations should be undertaken when carying out a simple acid-base titration

Answer 29

• wear eye protection and gloves
• if spilled immediately wash affected parts after spillage
• if substance is unknown treat it as potentiall toxic and wear gloves

Question 30

What must you consider when titrating mixtures to work out the concentration of the active ingredient

Answer 30

• it is necessary to consider wether the mixture contains other substances that have acid-base properties
• if they dont have acid-base properties you can titrate with confidence

Question 31

What should you do if testing batches for quality control

Answer 31

Titrate multiple different samples as the amount/concentration of chemical being tested may vary between samples

Question 32

What type of titration is manganate with iron and what is the colour change

Answer 32

• Self indicating
• colourless to pink/purple

Question 33

Describe how to determine how much iron is in iron tablets by titration

Answer 33

• Weigh accurately two ferrous sulphate iron tablets
• Grind up the tablets with a little 1 moldm^-3 sulfuric aicd using a pestle and mortar
• through a funnel transfer the resulting paste into a 100cm^3 volumetric flask.
• Use further small volumes of the 1 mol sulfuric acid to rise the ground up tablets into the flask
• Add sufficient 1 moldm^-3 sulfuric acid to make the solution up to 100cm^3
• Stopper the flask and shake it to make sure that all of the contents are fully mixed
• titrate 10cm^3 portions with 0.005 mol potassium manganate (VII).
• The end point is makred by ther first permanent purple colour.

Question 34

What is the ONLY acid appropriate for manganate titrations

Answer 34

Sulfuric acid

Question 35

What happens if insufficient volumes of sulfuric acid are used in a manganate titration

Answer 35

• The solution will not be acidic enough and MnO2 will be produced instead of Mn2+
• The brown MnO2 will masks the colour change and lead to a greater volume of manganate being used in the titration

Question 36

Why can ethanoic acid not be used in a manganate titration

Answer 36

• it is a weak acid so would not supple enough H+ ions
• This would lead to the brown MnO2 being produced
• This masks the colour change and leads to a greater volume of manganate being used in the titration.

Question 37

Why can concentrated hydrochloric acid NOT be used in a manganate titration

Answer 37

• the Cl- ions would be oxidised to Cl2 by MnO4- as seen by the EMF value for this reaction
• this would lead to a greater volume of manganate being used and poisonous Cl2 being produced

Question 38

Why can nitric acid NOT be used in a manganate titration with iron

Answer 38

• nitric acid is an oxidising agent. It oxidises Fe 2+ to Fe 3+ as shwon by the EMF values
• This would lead to a smaller volume of mangante being used.

Question 39

What is special about the titration between mangante ions and ethanedioate ions

Answer 39

it is autocatalysed by Mn2+

Question 40

Why is the initial rate of reaction between managante ions and ethanedoiate ions slow and what can be done to combate this

Answer 40

• It is slow to begin with as it is between two negative ions which repel each other
• you can heat the comcial flask to around 60 degrees to speed this up

Question 41

How do you work out the percentage uncertainty for each piece of equipment

Answer 41

% uncertainty= uncertainty/measurement made on apparatus x 100

Question 42

How do you work out the maximum/total percentage uncertainty for a practical procedure

Answer 42

Add all of the individual apparatus uncertainties together

Question 43

How do you decrease apparatus uncertainties

Answer 43

• decrease the sensitivity uncertainty by using apparatus with a greater resolution
• increase the size of the measurement made

Question 44

How do you reduce uncertainties in a titration

Answer 44

• replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the % uncertainty
• to reduce % uncertainty in the burette make the titre a larger volume

Question 45

What are two ways of making the titre a larger volume to decrease the percentage uncertainty in the burette

Answer 45

• increasing the volume and concentration of the substance in the conical flask
• decreasing the concentration of the substance in the burette

Question 46

How do you decrease uncertainties in measuring mass

Answer 46

• using a balance that will measure to more decimal places
• using a larger mass
• weighing the sample before and after addition then calculating the difference will ensure a more accurate measurement of the mass added