Techniques and Procedures

Question 1

Heating under reflux

Answer 1

Used for reactions involving volatile liquid. Ensures reactants and products do not escape in process.
Put reactants in pear shaped flask and add a few anti-bumping granules
Attach a condenser vertically to the flask so that the water flows from bottom to top
Heat so that reaction mixture boils gently using a Bunsen

Question 2

Purpose of anti bumping granules

Answer 2

Burst the bubbles in the boiling mixture and reduce the chance of it boiling over

Question 3

Don’t stopper flask in reflux. Why?

Answer 3

Would cause pressure to build up and the glassware to crack or stopper to fly out - cause serious accident

Question 4

Water must flow from bottom to top of condenser during reflux. Why?

Answer 4

Ensures condenser is always full of cold water

Question 5

purifying an organic liquid product using a separating funnel

Answer 5

when an organic product is mixed with another immiscible liquid (often aqueous liquid) separate using a separating funnel
the layers separate with the denser liquid forming the lower layer
allow the layers to separate
run off and dispose of aqueous layer
run the organic product layer into a clean conical flask

Question 6

purifying an organic liquid product when acidic impurities are present

Answer 6

add sodium hydrogen carbonate solution and shake

this removes acidic impurities

Question 7

purifying an organic liquid product by drying

Answer 7

dry the crude product by adding anhydrous sodium sulfate
swirl the mixture
(possible to use other drying agents eg. calcium chloride)

Question 8

purifying an organic liquid product by simple distillation

Answer 8

used to separate a mixture of miscible liquids with unique boiling points
obtain pure product
carried out after separation, removable of acidic impurities or drying

Question 9

making water-soluble inorganic salts - reacting acid and soluble base (alkali)

Answer 9

carry out acid -base titration to determine how much acid solution is needed to neutralise 25cm^3 alkaline solution
transfer 25cm^3 of the alkaline solution to a clean conical flask
using a burette add the correct amount of acid to neutralise the alkali
DO NOT USE INDICATOR
transfer neutralised solution to a clean evaporating basin and heat over a Bunsen flame to evaporate water
once crystals of solid start to appear stop heating
leave mixture to cool and then filter
wash solid residue with distilled water
transfer residue to watch glass and heat in an oven to dry solid
at regular intervals remove the watch glass and solid, cool in dessicator and weigh
stop process when solid reaches a constant mass

Question 10

making water-soluble inorganic salts - reacting acid and insoluble base

Answer 10

in a beaker, warm excess insoluble base in dilute acid
continue to warm (not boil) until solution is neutral, use indicator, adding more solid base if needed
leave to cool
filter off excess base and transfer filtrate to a dry clean evaporating basin
heat evaporating basin until salt crystals begin to appear
cool basin and filter mixture
discard filtrate and wash solid residue with distilled water
transfer residue to watch glass and heat in an oven to dry solid
at regular intervals remove the watch glass and solid, cool in dessicator and weigh
stop process when solid reaches a constant mass

Question 11

making water insoluble inorganic salts

Answer 11

prepared by reacting two solutions of soluble salts
add equal volumes of desired salt solution in a beak to form precipitate of insoluble salt
filter precipitate
wash precipitate several times with cold deionised water
transfer residue to watch glass and heat in an oven to dry solid
at regular intervals remove the watch glass and solid, cool in dessicator and weigh
stop process when solid reaches a constant mass

Question 12

process of simple distillation

Answer 12

put mixture into a pearshaped flask with antibumping granules
set up aparatus inc. condenser - water flowing from bottom to top
heat mixture until it boils gently using a bunsen flame/heating mantle
when vapour temperature is approximately two degrees below the boiling point of the liquid you are about to collect
put collecting beaker in place
collect distilled product until temp of vapour rises above the BP of the liquid you are collecting
stop heating

Question 13

thin layer chromatography

Answer 13

spot the test mixture and reference samples on a pencil line 1cm from the base of chromatography paper/silica plate
suspend plate in a beaker containing solvent - pencil line should be above level of solvent
cover beaker with watch glass to prevent evaporation of solvent
remove plate when solvent front is near the top
mark solvent front and allow plate to dry
locate any spots with iodine, ninhydrin or under UV ligbt
match height reached or Rf values with those of known compounds

Question 14

recrystillisation

Answer 14

select a solvent in which the desired substance is very soluble at higher temps and insoluble at lower temps
dissolve mixture in minimum quantity of hot solvent
filter under reduced pressurevto remove insoluble impurities and retain the filtrate
leave filtrate to cool until crystals form
collect crystals by filtering mixture under reduced pressure
dry crystals in an oven or air dry

Question 15

determining melting points of organic solids

Answer 15

seal the end of a glass melting point tube by heating it to melting in a bunsen flame
tap the open end of the tube into the solid so a small amount goes into the tube
tap the tube so the solid falls to the bottom of the sealed end
fix tube in melting point apparatus
heat surrounding liquid gently, temperature should rise very slowly
note temperature at which the solid starts and finishes melting - melting range
compare experimental value to published value - a pure compound will melt within 0.5 C of the true melting point

Question 16

making standard solution from a solid

Answer 16

calculate mass of solute required, and weigh out this amount accurately in a weighing bottle
pour 100cm^3 of deionised water into a 250cm^3 beaker
carefully transfer the weighed solute into the water and reweigh weighing bottle
stir mixture in beaker to ensure complete dissolving
transfer solution to a clean 250cm^3 volumetric flask
rinse beaker and stirring rod well with deionised water, adding rinsings to flask
use dropping pipette to add deionised water so that the bottom of the menisucs is level with mark on flask
stopper flask and invert it to ensure complete mixing

Question 17

calibrating pH electrode

Answer 17

wash electrode with distilled water
transfer electrode into a buffer solution of pH 7.00
wait for reading to stabilise
ensure meter reads 7.00, adjusting to that value if necessary

Question 18

calibrating pH electrode for acidic solutions

Answer 18

calibrate electrode using acid buffer solution pH 4.00

Question 19

calibrating pH electrode for alkaline solutions

Answer 19

calibrate electrode using alkaline buffer solution pH 10.00

Question 20

determining Ksp

Answer 20

prepare a saturated solution in deoionised water

warm distilled water in a small conical flask and add the salt you are looking to determine the Ksp of, shaking frequently
keep adding solute until no more dissolves
leave mixture to cool at room temp
there should still be solid at bottom of flask
filter the mixture through filter paper
discard residue

take temperature of the solutions you are working with
calculate concentration of one ion in the solution (by titration or colorimetry of a coloured solution)