practicals Flashcards
how would you remove the water of crystallisation
Remove as water vapour by heating in a crucible.
method for heating in a crucible
- weigh an empty dry and clean crucible and lid
- add the hydrated crystals and weigh again
- heat strongly with a bunsen for a couple of minutes
- allow to cool
- weigh the crucible and contents again
- heat again and reweigh until you reach a constant mass
why should you not use large amounts of the hydrated crystal in a ‘heating in a crucible’ experiment
as the decomposition is likely to be incomplete
why should you not use small amounts of the hydrated crystal in a ‘heating in a crucible’ experiment
as the percentage uncertainties when weighing would be too high
why should you use a lid when ‘heating in a crucible’
as it improves accuracy because it prevents loss of solid from the crucible. However, it should be loose fitting to allow any gasses to escape.
potential errors in using a gas syringe
- gas escapes before the bung is inserted
- syringe sticks
- some gases such as carbon dioxide are soluble in water so the true amount of gas might not be measured
making up a volumetric solution
- weigh the sample bottle containing the required mass of solid
- transfer to beaker
- reweigh sample bottle
- record the difference in mass
- add distilled water until dissolved. Use glass rod to stir
- if the substance does not dissolve well in cold water you could gently heat it
- pour into a 250cm3 graduated volumetric flask via a funnel
- rinse the beaker, glass rod and the funnel, adding the washings to the volumetric flask
- make up to the mark with distilled water using a dropping pipette when nearing the line
- invert flask several times to ensure the solution is uniform
why when u are making a solution do u invert the flat at the end
to make sure its a uniform solution / uniform concentration
what is a graduated flask
has one mark on the neck which the bottom of the meniscus should sit on.
why should u not heat the graduated flask or put hot liquids in it
as could cause the flask to expand and the volume would then be incorrect
why is a volumetric pipette more accurate than a measuring cylinder
it has a smaller uncertainty
titration general method
- rinse equipment
- pipette alkali into flat (25)
- Touch the surface of alkali with the pipette to ensure the correct amount it added
- add a few drops of indicator
- add acid solution from burette, making sure the jet space is filled with acid
- swirl mixture
what are concordant results
2 readings within 0.1 of each other
why use a conical flask over a beaker in a titration
it is easier to swirl the mixture without spilling
why do you touch the surface of the solution with the pipette during titrations
this ensures that the correct amount is added. Even after this a small amount of solution will be left in the pipette, however, the calibration of the pipette takes this into account so it should not be forced out.
why do you rinse the burette with the substance being put in it
incase there is any residual water in it or substances that may react, that could dilute / change the concentration of the substance in the burette and so a larger tittre would be delivered.
why should u not leave the funnel in the burette during a titration
as small drops of liquid might fall from the funnel during the titration leading to a false burette reading (a lower titre volume)
why should you make sure that the jet space is filled in a burette during a titration
as if it is not filled prior to starting the titration, it will lead to errors if it then fills up during the titration, leading to a titre reading that it higher than expected
how do you record a burette reading in a titration
even though it has markings that read to 0.1cm3, the readings should always be given to 2dpn either ending in 0.00 or 0.05. This is because the volume of drops are 0.05 and so this is the smallest difference in readings that can be measured.
why should u only add a few drops of indicator
they are usually weak acids so if too much is added it could effect the titration results as it reacts with the solution.
why might you add distilled water to the conical flask during a titration
This is to wash the sides of the flask so that all of the acid on the side is washed into the reaction mixture. it does not affect the titration readings as water doesn’t react with the reagants or change the number of moles of acid.
what should you wash the conical flask with between titrations and why
distilled water because it does not add any extra moles of reagents
give a specific example of a titration in which it is hard to see the bottom of the meniscus in the burette
if using manganate ions. The purple colour of the manganate can make it difficult to see the bottom of the meniscus in the burette.
what acid should you use for manganate titrations and why
Only use Dilute sulfuric acid.
The acid needs to supply the 8H+ ions so must be a strong acid.
It cannot set up alternative redox reaction eg.
conc HCl contains CL- ions which would be oxidised to CL2 by the MnO4- ion.
why would you not use conc HCl in a manganate redox titration
The cl- ions would be oxidised to Cl2 by the MnO4-n ions.
This is because the meg of MnO4-/Mn2+ > CL2/Cl-
This would lead to a greater volume of manganate being used and poisons Chlorine gas being produced
why would you not use conc nitric acid in a manganate redox titration
Nitric acid is an oxidising agent and would oxidise the Fe2+ to Fe3+ as
Eo NO3-/HNO2> Eo Fe3+/Fe2+
This would lead to a smaller volume of manganate being used.
errors in flame calorimetry
- energy losses from calirometry
- heat capacity of calorimeter not included
- incomplete transfer of energy
- incomplete combustion of fuel
- not carried out under standard conditions eg. H20 may be as a gas not liquid due to the heat
errors in calorimetry calculations
- energy transfer from surroundings
- approx heat capacity of solution as its assuming its the same as water
- its ignores that energy is absorbed by the apparatus
- reaction may be incomplete
- density of the solution is presumed to be the same as water
