DM - Formulae, equations and amount of substance *02 Flashcards
what are acid-base titrations used for?
to find how much acid is needed to neutralise a base
How do you set up a titration?
- measure out some alkali using a pipette and put it into a flask
- add indicator, phenolphthalein
- do a rough titration to get an idea of where the end point is
- add acid to alkali using a burette and swirl - do an accurate titration
- drop wise - record the vol. of acid used to neutralise the alkali
- repeat this until results are concordant (within 0.1cm^3 of eachother)
What are redox titrations?
used to find out how much oxidising agent is needed to exactly react with a quantity of reducing agent
What do you need to know when doing a redox calculation?
the conc. of either the reducing or oxidising agent
use titration value to work out conc. of the other
what redox titration do i need to know for the exam?
when manganate (VII) ions, MnO4- are the oxidising agent
method of redox titration using manganate (VIII) ions?
- measure vol. of reducing agent, (eg aq Fe2+ ions), using a pipette and put into a conical flask
- 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 - add aq MnO4- (oxidising agent) to reducing agent using a burette, swirling conical flask
- 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
- now do accurate titrations until readings are within 0.10cm^3 of each other (concordant)
how do you know when the redox titration with Manganate (VII) ions is complete?
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
How can you use titration results of find the conc. of a reagent?
- work out moles
- use mole ratio (in balanced equation) to work out moles in each reactant/product
- work out conc. (should be given vol. values)
concentration = moles / volume