Chemistry required practicals

• Safety precaution when using acid, alkali

  wear goggles
• Safety precaution when using alcohols

  -flammable, keep away from naked flame

  -water bath, electric heater
• Safety precaution when using something toxic

  -wear gloves

  -avoid contact

  -wash hands after use
• Safety precaution when using something that is oxidising

  -keep away from flammable, easily oxidised materials
• Safety precaution when using hazardous substances

  use in small concentrations
• Heating in a crucible

  -measure mass loss in thermal decomposition reaction and also mass gain when reacting with O2
• Water of crystallisation

  -remove water by heating
• Method for water of crystallisation

  -weigh empty crucible and lid

  -weigh substance with crucible and lid

  -heat with bunsen burner, then cool

  -weigh again

  -heat again until mass constant
• Why do we weigh lid of crucible?

  -improve accuracy, prevent loss of solid from crucible, but should be loose fitting to allow gas to leave
• Why shouldn't large quantities used for water of recrystallisation?

  -decomposition incomplete

  -percentage uncertainties in weighing will be high
• Why does crucible need to be dry?

  if wet, inaccurate result, mass loss will be too large as water is lost when heating
• Why are gas syringes used?

  measure volume of gas, work out moles of gas or to follow reaction rate
• What does volume of gas collected from gas syringe depend on?

  pressure, temperature, so important to note down temp and pressure when measuring volume of gas
• Potential errors when using gas syringe

  -gas escape before bung put on

  -syringe sticks

  -some gases like carbon dioxide and sulphur dioxide are soluble in water so not measured
• Method to measure and calculate Mr of propanone

  -extract volume into hypodermic syringe and measure mass of syringe

  -using hand protection, remove gas syringe out of oven, note volume of air in the barrel

  -inject propanone

  -put the gas syringe back into oven

  -measure mass of empty hypodermic syringe

  -measure vol of gas, record temp of the oven and pressure of the room
• making standard solution

  -weigh empty weighing boat, add solid, reweigh

  -add solid to beaker

  -calculate difference in mass

  -add distilled water, dissolve with stirring rod

  -add to graduated flask, using funnel

  -rinse beaker, stirring rod, funnel add to beaker

  -make up to 250 cm3, add stopper, invert to mix
• Why shouldn't you heat graduated flask or add hot solutions?

  Flask only have one mark on the neck, heat would cause flask to expand so volume would be inaccurate
• How to measure mass correctly?

  -measure mass of sample bottle

  -add sample to beaker

  -reweigh sample bottle

  -note measurements and calculate difference in mass
• Dilutions

  -pour 25 cm3 of one solution into volumetric flask

  -make up mark with distilled water, use dropping pipette for last few drops

  -invert flask to mix
• Why is volumetric pipette more accurate than measuring cylinder?

  -small uncertainty
• which solution is in the conical flask in titration?

  -conical flask with unknown concentration
• 'titrate A with B'

  A is in the conical flask, B is in the burette
• Method for titration

  -Rinse equipment

  -pipette 25cm3 of alkali into conical flask

  -fill burette with acid

  -make sure jet space is filled with acid

  -add indicator, use white tile

  -add acid to alkali whilst swirling the mixture, add dropwise towards end point

  -note burette reading before and after

  -repeat until concordant result
• Why is conical flask used instead of beaker for titration?

  easier to swirl, without spilling contents
• Distilled water can be added to which solutions when titrating?

  -solution in conical flask

  -doesn't affect titration reading as water does not react with reagent or change moles of substance added
• Ions in manganate redox titration

  Fe2+ and MnO4-
• How much iron is in an iron tablet using manganate redox?

  (titration method)

  -weigh tablet

  -grind with sulphuric acid, use mortar and pestle

  -use funnel, transfer resulting paste into 100cm3 volumetric flask

  -add sulphuric acid to rinse mortar and pestle add to flask

  -add more H2SO4 to make up to 100cm3 mark

  -add stopper, shake

  -titrate 10 cm3 with potassium manganate
• How many H+ ions need to be supplied for the manganate titration?

  8
• Why is only H2SO4 being used for manganate redox titration?

  insufficient amounts of acids will form MnO2 instead of Mn2+, brown MnO2 masks colour change, lead to greater volume of manganate being used in titration
• Why can't HCl be used in manganate redox titration?

  Cl- oxidised to Cl2 by MnO4- as E cell MnO4- > E cell Cl-

  produce poisonous Cl2 ions
• Why can't nitric acid be used in manganate redox titration?

  -oxidising agent, oxidises Fe2+ to Fe3+

  E cell of NO3- > E cell of Fe3+

  -smaller of volume of manganate used
• How to reduce uncertainty in titration?

  -replace measuring cylinder with pipettes or burettes

  -in burette reading, increase volume and concentration of substance in conical flask or decrease concentration of substance in the burette
• How to know when result is due to other errors other than apparatus?

  -if uncertainty due to apparatus is lesser than percentage difference between actual value and calculated value
• How to know when error is due to apparatus and equipment?

  when uncertainty by apparatus is larger than percentage difference between actual and calculated value then there is no discrepancy and all errors are due to equipment