DM - Formulae, equations and amount of substance *02

Question 1

what are acid-base titrations used for?

Answer 1

to find how much acid is needed to neutralise a base

Question 2

How do you set up a titration?

Answer 2

1. measure out some alkali using a pipette and put it into a flask
2. add indicator, phenolphthalein
3. do a rough titration to get an idea of where the end point is
   - add acid to alkali using a burette and swirl
4. do an accurate titration
   - drop wise
5. record the vol. of acid used to neutralise the alkali
   - repeat this until results are concordant (within 0.1cm^3 of eachother)

Question 3

What are redox titrations?

Answer 3

used to find out how much oxidising agent is needed to exactly react with a quantity of reducing agent

Question 4

What do you need to know when doing a redox calculation?

Answer 4

the conc. of either the reducing or oxidising agent

use titration value to work out conc. of the other

Question 5

what redox titration do i need to know for the exam?

Answer 5

when manganate (VII) ions, MnO4- are the oxidising agent

Question 6

method of redox titration using manganate (VIII) ions?

Answer 6

1. measure vol. of reducing agent, (eg aq Fe2+ ions), using a pipette and put into a conical flask
2. add some dilute sulfuric acid to the flask (in excess)
   - acid makes sure that there are plenty of H+ ions to allow the oxidising agent to be reduced
3. add aq MnO4- (oxidising agent) to reducing agent using a burette, swirling conical flask
4. stop when the mixture just becomes tainted with the colour of MnO4- (the end point) and record the vol. of the oxidising agent added - this is a rough titration
5. now do accurate titrations until readings are within 0.10cm^3 of each other (concordant)

Question 7

how do you know when the redox titration with Manganate (VII) ions is complete?

Answer 7

Manganate (VII) ions in aq potassium manganate (VII) - KMnO4 - are PURPLE

when they are added to the reducing agent they start reacting with the reducing agent, they get reduced to COLOURLESS Mn2+ ions.

the reaction continues until all of the reducing agent has reacted

the very next drop will give the mixture a PINK colour of the oxidising agent

need to spot exactly when this happens

Question 8

How can you use titration results of find the conc. of a reagent?

Answer 8

1. work out moles
2. use mole ratio (in balanced equation) to work out moles in each reactant/product
3. work out conc. (should be given vol. values)

concentration = moles / volume