Titration

Question 1

what’s the general method for a titration

Answer 1

• rinse equipment
• pipette 25cm3 of alkali into conical flask
• touch surface of alkali with pipette
• add acid solution from burette
• make sure the jet space in the burette is filled with acid
• add a few drops of indicator and refer to colour change at end point
• phenolphthalein ( pink to colourless ) ( use if NaOH is used)
• methyl orange ( yellow to red) ( use if HCl is used)
• use a white tile underneath the flask to help observe the colour change
• add acid to alkali whilst swirling the mixture and add acid drop wise at end point
• note burette reading before and after addition of acid
• repeat titration until at least 2 concordant results are obtained - two results within 0.1 of each other

Question 2

where is the substance placed where we don’t know the concentration

Answer 2

in the conical flask .
measured using a volumetric pipette
Other substance is placed in the burette

Question 3

why is a conical flask used instead of a beaker

Answer 3

it’s easier to swirl the mixture in a conical flask without spilling the contents

Question 4

what’s the method for using the pipette

Answer 4

• rinse pipette with substance to go in it
• pipette 25cm3 of solution A into conical flask
• bottom of meniscus should sit on this line
• touch surface of solution with pipette

Question 5

what’s the method for using the burette

Answer 5

• should be rinsed out with substance that will be put in it
• if not rinsed the acid or alkali added may be diluted by residual water or may react with substances left from a previous titration .
• this would lead to the conc of the substance being lowered and a larger titre
• don’t leave funnel in burette because small drops or liquid may fall from funnel during titration leading to a false burette reading ( smaller titre volume )
• make sure jet space in burette is filled with solution and air bubbles are removed

Question 6

what happens if the jet space in the burette is not filled properly before the titration

Answer 6

it will lead to errors if it then fills in the titration , leading to a larger tire reading

Question 7

why would we add distilled water to the conical flask during a titration to wash the sides of the flask

Answer 7

so all the acid on the side is washed into the reaction mixture to react with the alkali
- it does not affect the titration reading as water does not react with the reagents or change number of moles of acid added

Question 8

what are some safety precautions of titrations

Answer 8

acids and alkalis are corrosive
wear eye protection and gloves

Question 9

what’s the equation for manganate redox titrations

Answer 9

MnO4-(aq) + 8H+ (aq) + 5Fe2+ (aq) => Mn2+(aq) + 4H2O(l) + 5Fe3+(aq)

Question 10

what’s the colour change in the manganate redox titrations

Answer 10

colourless to pink

Question 11

what’s the method to find out how much iron is in the iron tablets

Answer 11

• weigh accurately two iron tablets
• grind up the tablets with a little 1moldm-3 sulphuric acid , using pestle and mortar.
• through a funnel , transfer the resulting paste into a 100cm3 volumetric flask .Use a further small volumes of 1moldm-3 sulphuric acid to rinse the ground up tablets into the flask
• then add sufficient 1moldm-3 sulphuric acid to make up the solution to exactly 100cm3 . stopper the flask and shake it to make sure all the contents are thoroughly mixed . They will not all be in solution although Fe2+ ions which were present in the tablets will be dissolved .
• Titrate 10cm3 portions of the solution with the 0.0050moldm-3 potassium manganate . end point is marked by the first permanent purple colour

Question 12

what acid would you use for manganate titrations

Answer 12

only use dilute sulphuric acid

Question 13

what would be produced instead of Mn2+ if insufficient volumes of sulphuric acid is added and what’s the equation

Answer 13

MnO2
MnO4- (aq) + 4H+ (aq) + 3e- => MnO2(s) + 2H2O

Question 14

why can’t weak acids be used in manganate titrations

Answer 14

it cannot supply the large amount of hydrogen ions needed (8H+)

Question 15

why can’t a concentrated strong acid such as HCl be used in a manganate titration

Answer 15

The Cl- ions would be oxidised to Cl2 by MnO4- because the Emf value for MnO4-/Mn2+ is bigger than the EMF of Cl2/Cl-
This would lead to a greater volume of manganate being used and poisoned Cl2 being produced

Question 16

why can’t nitric acid be used in a manganate titration

Answer 16

It’s an oxidising agent .
It oxidises Fe2+ to Fe3+ as EMF for NO3- is bigger than Fe2+ / Fe3+
This would lead to a smaller volume of manganate being used

Question 17

EQ : give two reasons why it’s essential to avoid this reaction ( MnO4- to MnO2) in a titration between potassium manganate and iron ions . (2)

Answer 17

large titre than expected
cant see end point due to brown colour

Question 18

EQ : Stare the colour change that occurs at the end point of the titration of MnO4- and why (2)

Answer 18

Colourless
just after end point MnO4- is in excess of

Question 19

why might adding too much indicator be a problem and how can we fix it

Answer 19

It may react and effect the endpoint reading
- use a smaller volume (2-6 drops)

Question 20

Why is using a measuring cylinder not good for measuring acid in a titration and what can we use instead

Answer 20

it has a large uncertainty
Use a volumetric pipette

Question 21

Why is rinsing a burette with deionised water before adding alkali not a good idea and explain improvements

Answer 21

• the. deionised water will slightly dilute alkali solution
• rinse burette with alkali solution

Question 22

why shouldn’t you wait until solution “just” changes colour in a titration and what can you do to improve it

Answer 22

acid may not have fully reacted with the alkali
- add alkali solution until permanent colour change is seen

Question 23

give a reason for the colour change of manganate redox reaction

Answer 23

Caused by excess MnO4-