PRACTICALS Flashcards
required prac 1 - making a standard solution method
- Weigh the sample bottle containing the required mass of solid
on a 2 dp balance - Transfer to beaker * Reweigh empty sample bottle
- Record the difference in mass
- Add 100cm3 of distilled water to the beaker. Use a glass rod to
stir to help dissolve the solid.
*Sometimes the substance may not dissolve well in cold water so
the beaker and its contents could be heated gently until all the
solid had dissolved. - Pour solution into a 250cm3 graduated flask via a funnel.
- Rinse beaker and funnel and add washings from the beaker
and glass rod to the volumetric flask. - make up to the mark with distilled water using a dropping
pipette for last few drops. - Invert flask 10 times to ensure uniform solution.
required prac 1 - titration
- rinse the equipment with the solutions they will contain (burette with acid, volumetric pipette with alkali, conical flask with distilled water)
- use a volumetric pipette to transfer 25cm3 of alkali (unknown conc.) into a 250cm3 conical flask + touch surface of alkali with pipette (to ensure correct amount is added)
- add standard/acid solution (known conc.) to burette
- add a few drops of indicator to conical flask and refer to colour change at end point
- add acid to alkali slowly whilst swirling the mixture and add acid dropwise near endpoint (until indicator undergoes a definite, permanent colour change)
- note burette reading before + after addition of acid
- rep - until get 2 concordant results - within 0.10
what happens if you dont rinse the titre - req prac 1
- if not rinsed: acid/alkali added may be diluted by residual water/react with substances from previous titration = lower conc. of substance and larger titre
why are volumetric pipettes and concial flasks used in titration - rp 1
- volumetric pipette = smaller uncertainty
- conical flask = easier to swirl without spillage
why make sure that the tap/jet space is filled before starting the titration -rp 1
- make sure jet space in the burette is filled with acid and does not contain air bubbles (leads to errors: larger than expected titre reading)
why use a funnel in a titration -rp1
- use funnel (at eye level) to prevent spillage
- don’t leave the funnel in the burette as small drops of liquid may fall from the funnel during titration (leads to a false burette reading (gives a lower titre volume))
what indicators are requred for each titration and why
- only a few drops: generally weak acids so too much will affect titration result
- phenolphthalein (strong alkali e.g. NaOH): colourless in acid, pink in alkali, endpoint = colourless <=> pink
- methyl orange (weak alkali e.g. NH3): red in acid, yellow in alkali, endpoint = orange
how to ensure accuracy of the titre results - rp1
- make sure all burette readings are to the appropriate precision and are read from the
bottom of the meniscus - note titre volume (change in volume in the burette) to 2 dp (0.05cm3)
-to find the mean titre - only get average from the concordant ones
-read to either - 0.05 - if in the middle - or 0.00 not inbwt
how to reduce uncertainty in a titre
Replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the error.
-To reduce the uncertainty in a burette reading it is necessary to make
the titre a larger volume. This could be done by: increasing the volume
and concentration of the substance in the conical flask or by decreasing
the concentration of the substance in the burette.
why can distilled water be added to the titre
-Distilled water can be added to the conical flask during a titration to wash the sides of the flask so that 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 the number of moles of acid added.
give some common titration equations
CH3CO2H + NaOH = CH3CO2-Na+ + H2O
H2SO4 + 2NaOH = Na2SO4 +2H2O
HCl + NaOH = NaCl +H2O
NaHCO3 + HCl = NaCl + CO2 + H2O
Na2CO3 + 2HCl = 2NaCl + CO2 + H2O
required practicle 1 - finding out how much iron is in iron tablets
- Weigh accurately two ‘ferrous sulphate’ tablets. * Grind up the tablets with a little 1M sulphuric acid, using a pestle and mortar.
- Through a funnel, transfer the resulting paste into a 100cm3 volumetric flask. Use further small volumes of 1 M sulphuric acid to rinse the ground-up tablets into the flask.
- Then add sufficient 1M sulphuric acid to make up the solution to exactly 100cm3. Stopper the flask and shake it to make sure that all the contents are thoroughly mixed. They will not all be in solution although the Fe2+ ions which were present in the tablets will be dissolved.
- Titrate 10.0 cm3 portions of the solution with 0.0050 M potassium manganate(VII). The end-point is marked by the first permanent purple colour
which acid must be used for titration of manganate ion/iron tablet
-dilute sulphuric acid
why can nitric acid not be used as the acid in iron tablet titration
-It cannot be nitric acid as it is an oxidising agent. It oxidises Fe2+ to Fe3+ as
E NO3-/HNO2> Eo Fe3+/Fe2+
NO3- (aq) + 3H+(aq) + 2e– = HNO2 (aq) + H2O(l) E +0.94V
Fe3+ (aq)+e– Fe2+ (aq) E +0.77 V
This would lead to a smaller volume of manganate being used.
why cant conc HCl be used as the acid for the iron tablet titration
Cl- ions would be oxidised to Cl2 by MnO4- as the E MnO4 -/Mn2+ > E Cl2/Cl- MnO4
-(aq) + 8H+ (aq) + 5e– Mn2+ (aq) + 4H2O(l) E+1.51V
Cl2(aq) +2e– = 2Cl–(aq) E +1.36V
This would lead to a greater volume of manganate being used and poisonous Cl2 being produced.
why is using weak acids/ insufficent volumes of sulphuric acid not useful for titration of iron tablets
Insufficient volumes of sulphuric acid will mean the solution is not acidic enough and MnO2 will be produced instead of Mn2+
MnO4 -(aq) + 4H+ (aq) + 3e- MnO2 (s) +2H2O
The brown MnO2 will mask the colour change and lead to a greater (inaccurate) volume of Manganate being used in
the titration.
Using a weak acid like ethanoic acid would have the same effect as it cannot supply the large amount of hydrogen
ions needed (8H+).
