A level CPACS

Question 1

CAPC 9: Finding Ka
- Why is a pH probe calibrated
- How to calibrate a pH probe
- Indicator used

Answer 1

• accurate reading
• Calibrating in ph7,4,10 solutions
• phenolphthalein

Question 2

CPAC 10: E cell
- What salt is used for the salt bridge
- Why are graphite/Pt used for electrodes
- How is EMF measured
- Why would a metal be filed before calculating E cell

Answer 2

• Potassium nitrate used for salt bridge
• graphite/Pt used bc they are inert
• EMF measured by OA-RC
• file outer layer of metal to remove oxide

Question 3

CPAC 11: Redox titration

Answer 3

• KMnO4 in burette
• stop at permanent pale pink colour

Question 4

CPAC 12: Metal complex preparation
- Why is conc NH3 added in a fume hood
- How are the crystals initially formed
- How to purify/isolate the crystals
- How to increase the yield of the crystals

Answer 4

• conc NH3 gives off toxic fumes
• cooled in ice bath
• vacuum filtration
• wash the beaker to avoid losing solid

Question 5

CPAC 13: Iodine clock reaction
- Reactants
- Why are samples taken
- What is used to quench the reaction
- How to measure when experiment is complete
- Why does solution turn blue black

Answer 5

• propanone, H2SO4, iodine used
• whole reaction cannot be stopped - measuring conc at a specific point
• NaHCO3
• measure how long it takes for blue-black colour to appear
• when there are no more thiosulfate ions present to react with I2

Question 6

CPAC 14: Arrhenius equation with bromide and bromate(V)
- why is a log scale used
- what indicator is used

Answer 6

• log scale used for large range in data
• add bromide, bromate(V) and phenol
• methyl red used as indicator
• add H2SO4, measure time taken for colour to disappear

Question 7

CPAC 14: Arrhenius equation with bromide and bromate(V)

Answer 7

• log scale used for large range in data
• add bromide, bromate(V) and phenol
• methyl red used as indicator
• add H2SO4, measure time taken for colour to disappear

Question 8

CPAC 16: Making aspirin
- Reactants
- Purification method

Answer 8

• salicylic acid & ethanoic anhydride with H2SO4 catalyst
  Purification:
  1. dissolve in minimum volume of hot solvent to remove insoluble impurities
  2. filter to remove insoluble impurities
  3. use ice to cool and recrystallise
  4. wash with cold solvent to remove impurities on crystals