Chemistry Required Practicals: Methods

Making a Standard Solution

• Ensuring a stock solution of known volume and concentration

• Measure _g of solute on a weighing boat

• Wash the solute into a beaker

• Add some water and use a stirring rod to completely dissolve solute

• Pour the solution into a volumetric flask and do washings of the beaker, rod and funnel into the volumetric flask

• Add distilled water until it reaches the graduation line

• Put in the stopper and do multiple inversions

Titration

• Determining unknown concentrations

• Using a glass pipette, place 25cm³ of the solution to be analysed in a conical flask

• Add a few drops of a suitable indicator

• Fill a burette with the standard solution

• Open the tap to add the solution until the indicator just shows a colour change. Note down the volume of solution used (to the nearest 0.05cm³)

• Repeat until you reach concordant results. Take a mean of the concordant results

Back Titrations

• Determining unknown masses

• Used for acids / bases which are not very soluble in water or react slowly

• Add a known volume and concentration of an acid / alkali in excess to the sample

• Titrate the left over acid / alkali

Heating to Constant Mass

• Determining rate of reaction

• Heat reaction mixture

• Measure mass multiple times over a range of time

• Once the mass stops changing, the reaction has stopped

Finding the Mr of a Volatile Liquid

• Vaporise a known mass of liquid in at a known temperature and pressure

• The liquid is injected from a hypodermic syringe into a gas syringe

• Measure the volume and then use the ideal gas equation

Calorimetry

• For determining the enthalpy change (of combustion)

• The heat released / absorbed by a chemcial reaction is measured

• The heat released warms up a known mass of water

• Water’s specific heat capacity is knwon

• The reaction ins done in a polystyrene cup

• q = (mass of water) x (4.18) x (change in temp) is used

• then, enthalpy change = -(q / 1000) / mol of substance

Reflux

• For heating reactants at a constant temperature

• Reactants are put in a Liebig Condenser

• Antibumping granules are added

• The water jacket is connected to a water source

Simple Distillation

• For separating a mixture of liquids

• Place mixture in round-bottomed flask

• Turn on water supply for water jacket

• Heat to boiling point of desired product

Filtration under Reduced Pressure

• Separating solid from liquid

• Attach water pump

• Ensure vacuum is produced

• Place filter paper in the Buchner funnel

• Place solid (crystals) on the filter paper

• Wait until liquid is removed

Purification

• Removing impurities

• Liquid collected from reaction is placed in separating funnel

• Water is added to dissolve water-soluble impurities

• Two layers will form

  • The less dense organic layer

  • The more dense aqueous layer

• The organic layer is removed from the separating funnel

• A drying agent is added to remove any remaining water

• The organic layer is filtered from the drying agent

Recrystallisation

• Removes insoluble impurities

• Dissolve crystals in minimum volume of hot solvent

  • Product must be more soluble in got solvent compared to cold solvent

• Filter hot solution using hot glassware and fluted filter paper

• Solution is allowed to cool at room temperature

• Product is filtered under reduced pressure with Buchner apparatus

• Products is washed with small amount of cold solvent

• Product is dried with filter paper

Using a Colourimeter

• Measure the concentration of a substance as it is proportional to the absorbance

• Standardise the colourimeter by measuring absorbance of distilled water in a cuvette

  • The value should be 0

• Measure the absorbance of difference concentrations and record the value

• A graph with a line of best fit can be used to determine an unknown concentration

Thin Layer Chromatography

• Separate a mixture

• Draw a pencil line as the origin

• Add a concentrated dot of solution on the origin line

• Place the silica plate in the solvent, ensuring the origin line lies above the solvent surface

• Let the solvent move up the plate until it reaches near the top

• Draw a line where the solvent front it

• Leave plate to dry

• Measure Rf values

Measuring EMF

• In a beaker place a strip of metal and the metal ions

  • Eg copper and copper chloride

• Do the same for a different metal in a second beaker

• Soak a strip of filter paper in KNO₃

• Use the filter paper as a salt bridge

• Connect crocodile clips to each metal strip and a voltmeter

• Measure the potential difference