Techniques and Procedures

Question 1

how to weigh a solid

Answer 1

1. zero the balance / tare the balance
2. place a weighing bottle on the balance and add in approximately the required mass of solid
3. accurately weigh the required mass of solid and record
4. empty the solid into the glassware
5. accurately re-weigh the empty weighing bottle
6. subtract the mass of the empty weighing bottle from the bottle + solid mass

Question 2

what pieces of equipment are used to measure liquids

Answer 2

• burette
• volumetric pipette

Question 3

how to measure volume of a liquid using volumetric pipette

Answer 3

1. rinse pipette with distilled water then a small volume of liquid to be measured with
2. dip pipette into solution and draw up solution until the bottom of the meniscus is exactly on the line at eye level with pipette line
3. run liquid out of the pipette into the glassware
4. allow liquid to run out of the pipette until it stops and gently touch bottom of pipette on glassware

Question 4

how to measure volume of liquid using a burette

Answer 4

1. use a clamp stand to hold up the burette
2. rinse burette with distilled water, then with a small amount of liquid to be measured
3. ensure burette tap is closed, fill burette with desired solution using a small funnel until above the 0 line
4. allow some of the solution to run from burette to the beaker to fill the tap and record the reading accurately to the nearest 0.05 cm3
5. carry out titration to the end point
6. record final reading on the burette to the nearest 0.05 cm3
7. subtract final reading from the original reading, record this as the titre

Question 5

how can gas be collected and measured

Answer 5

1. using a gas syringe
2. using an inverted measuring cylinder (collecting gas over water)

Question 6

when do you heat over reflux

Answer 6

used for reactions including volatile liquids, it ensures no reactions and products escape the reaction while it is in progress

Question 7

how to heat under reflux

Answer 7

1. put reactions into pear shaped flash and add a few anti-bumping granules (reduce the chance of it boiling over)
2. do not stopper the flask
3. attach a condenser vertically to the flask so water flows into the condenser at the bottom and flows out at the top
4. heat so that the reaction boils gently, using a bunsen burner, any vapours should not reach more than halfway up the condenser before condensing back into liquid

Question 8

why do we not stopper the flask during reflux

Answer 8

this would cause the pressure to build up and the glassware would crack or the stopper would fly out. A serious accident would result.

Question 9

why does water always flow into the bottom of the condenser and out at the top
(during reflux)

Answer 9

it ensures the water in the condenser is always cold

Question 10

why do we purify organic liquids

Answer 10

to remove any solvents or impurities present

Question 11

how to purify an organic liquid

Answer 11

1. allow the organic liquid to settle, the liquid will separate into layers with the denser at the bottom and the less dense on top
2. run off and dispose of the aqueous layer and run the organic product layer into a clean conical flask
3. acid impurities - add sodium hydrogen carbonate and shake well
   alkaline impurities - add dilute HCl until mixture is neutral
4. dry crude product by adding anhydrous sodium sulfate swirl mixture (or calcium chloride)
5. then separate product by distillation

Question 12

what are the two ways you can make water-soluble salts

Answer 12

1. reacting an acid with a soluble base
2. reacting an acid with an insoluble base

Question 13

how to make water-soluble salts by reacting acid with soluble base

Answer 13

this is done by acid base titrations
1. transfer 25 cm3 of alkaline solution into a clean conical flask
2. using a burette, add the correct amount of acid to neutralise the alkali, do not add indicator
3. transfer the neutralised solution into a clean evaporating basin and heat over a bunsen flame to evaporate the water, stop heating when crystals appear
4. leaving mixture to cool
5. filter mixture
6. wash solid residue with distilled water
7. transfer solid residue to watch glass and heat in oven a temperature below melting point
8. weigh until watch glass and solid reaches a constant mass and cool in dessicator

Question 14

how to make water soluble organic salts by reacting an acid with an insoluble base

Answer 14

1. warm warm excess insoluble base in dilute acid
2. continue to warm until solution is neutral
3. filter off the excess base and transfer filtrate to a clean dry evaporating basin
4. heat evaporating basin until salt crystals appear on the sides of the basin
5. cool basin the filter mixture
6. wash solid with cold distilled water
7. transfer residue to wath glass and heat gently and below melting point
8. heat until constant mass and allow solid to cool in desiccator

Question 15

how to you make water-insoluble inorganic salts

Answer 15

1. add equal volumes of desires salt solutions in a beaker to form a precipitate of insoluble salt
2. filter off the precipitate
3. wash precipitate 3 times in deionised water
4. transfer filtered, washed, precipitate to clean watch glass and place in drying oven set below melting point
5. weigh until mass is constant, then cool is desiccator

Question 16

what are the methods of purification

Answer 16

1. simple distillation
2. thin layer chromatography
3. paper chromatography
4. recrystallisation
5. vacuum filtration

Question 17

how to carry out simple distillation and WHEN do we use it

Answer 17

• used to separate liquid with different boiling points
  1. put mixture into pear shaped flask and add anti bumping granules, set up equipment as in diagram (DRAW DIAGRAM)
  2. heat mixture until it boils gently using bunsen burner with orange flame
  3. collect the liquid until the temperature rises 2 degrees of boiling point of desired liquid
  4. stop heating

Question 18

how to carry out thin layer or paper chromatography and WHEN to use them

Answer 18

• to separate small quantities of organic products or purify organic products
  1. spot the test mixture and reference samples on a light pencil line 1 cm above the base of the chromatography paper or silica plate
  2. suspend the plate in the beaker containing the solvent and cover the beaker with a watch glass to prevent it from evaporating
  3. remove plate when solvent is near the top
  4. locate any spots with iodine, ninhydrin or an ultra violet light
  5. match the heights reached with reference spots or calculate Rf values and mach them with database

Question 19

how to carry out recrystallisation and when to use it

Answer 19

• used to purify solid crude organic products with a small amount of impurities
  1.suitable hot solvent is chosen that dissolves only the required compound and at high temperatures
  2. dissolve the mixture in minimum quantity of hot solvent, smaller the amount, the greater the yield
  3. preheat filter flask and conical flask to prevent any desired solid recrystallising, filter to remove any insoluble impurities and retain the filtrate
  4. leave filtrate to cool until crystals form
  5. collect crystals by vacuum filtration
  6. dry crystals in oven on low heat

Question 20

how to carry out vacuum filtration and when to use it

Answer 20

• used to separate a solid from a from a filtrate rapidly
  1. connect conical flash to vacuum pump via side arm
  2. dampen a piece of filter paper and place it flat in the buchner funnel
  3. switch on the vacuum pump then carefully pour mixture into buchner funnel to be filtered
  4. disconnect pump from the vacuum pump before turning the pump off

Question 21

how do you determine the melting points of organic solids

Answer 21

1. seal end of glass melting point tube by heating in flame and lightly pinching
2. tap open end of tube into a the solid so a small part moves into it, tap tube so solid falls to the bottom end
3. DRAW APPARATUS - b test tube with bong and thermometer attached to melting point tube all surrounded by suitable liquid
4. heat liquid gently
5. note the temperature at which the solid starts and finishes melting, difference between them = melting range
6. compare experimental value with published value and should be within 0.5 degrees of true melting point

Question 22

why do we make standard solutions

Answer 22

• one where its concentration is accurately known, that is used to determine concentration of an unknown

Question 23

how to make a standard solution from a solid
- assume its being made up to 250 cm3

Answer 23

1. calculate mass of solute required, weigh this amount accurately in a weighing bottle to the nearest 0.01g, note mass of weighing bottle and solute
   2.pour 100 cm3 deionised water into 250 cm3 beaker, transfer solute from weighing bottle into water
2. reweigh weighing bottle and record, find the difference in mass before and after tipping out solid
3. stir mixture in beaker
4. transfer solution into 250cm3 volumetric flask, rinse beaker and stirring rod with deionised water into volumetric flask, fill up flash up to 1 cm away from line
5. make up solution with deionised water and a dropping pipette until the bottom of the meniscus is on the line
6. insert stopper in the flask and invert 10 times

Question 24

how to make a standard solution from dilution
- assume you want to produce a 0.1 mol/dm3 solution from a standard solution of 1.0 mol/dm3
- assume the solution is being made up to 250 cm3

Answer 24

1. rinse the clean, dry beaker with stock solution then half fill it
2. use a pipette filler to rinse a clean 25 cm3 pipette with stock solution, then fill pipette with stock solution until bottom of meniscus is on the line
3. run the solution into a 250 cm3 volumetric flask
4. add deionised water to the solution, swirling at intervals to mix the contents until solution is 1 cm3 away from the mark
5. use a dropping pipette to add deionised water to solution until the bottom of the meniscus lies on the line
6. insert the stopper and invert 10 times

Question 25

what equation do you use to calculate the volumes you need when making a standard solution

Answer 25

C1 X V1 = C2 X V2
C1 - concentration of dilute solution being made
V1 - volume of dilute solution being made
C2 - concentration of stock (original) solution
V2 - volume of stock (original) solution
conc. mol/dm3
vol. dm3

Question 26

what is an acid base titration used for

Answer 26

• to calculate the concentration of an acid or alkali accurately

Question 27

how to carry out an acid-base titration
- assume you are titrating the concentration of a known alkali against an acid of unknown concentration

Answer 27

1. rinse burette with acid solution, the fill with acid and let some run through the tap into the beaker, record the initial reading of burette to the nearest 0.05
2. fill a clean 25 cm3 pipette with some of the alkaline solution, ensuring bottom of meniscus lies on the line
3. run alkaline solution into a clean 250 cm3 conical flask
4. add 3 drops of suitable indicator
5. run acid from burette into the flask whilst swirling, watch for the first hint of colour change, and slow down the release of acid until colour change
6. record final reading of burette to nearest 0.05. initial reading - final reading = titre. use this as your rough titre
7. Refill the burette and record the initial reading
   8.use pipette to transfer 25 cm3 of alkali to clean conical flask, add 3 drops of indicator
8. run acid solution to 1 cm3 below rough titre, then add drop wise until colour change.
9. repeat steps 6, 7 and 8 until you have concordant results (titres within 0.10 cm3 of each other)

Question 28

why are iodine-thiosulfate titrations used

Answer 28

• to find the concentration of a chemical that is strong enough to oxidise iodide ions to iodine.
• liberated iodine is titrated titrated with thiosulfate ions, with starch as an indicator

Question 29

step 1 of an iodine thiosulfate titration
- assuming you want to find the concentration of sodium chlorate (I)

Answer 29

1. pour some chlorate (I) solution into a dry beaker
2. rinse 25 cm3 volumetric pipette with water, then with chlorate (I) solution
3. transfer 25 cm3 chlorate (I) solution to a conical flask using a volumetric pipette and pipette filler ensuring the bottom of the meniscus is on the mark, touch the tip of pipette on bottom conical flask
4. measure 15 cm3 of 0.5 mol/dm3 potassium iodide to measuring cylinder (calculating amount of liberated iodide ions so doesn’t have to be precise)
5. measure 20 cm3 1.0 mol/dm3 sulfuric acid for excess hydrogen ions
6. contents of flask should be brown due to iodine

Question 30

step 2 of iodine-thiosulfate solution
- assuming you want to find the concentration of sodium chlorate (I)

Answer 30

• titrating solution with sodium thiosulfate to work out moles of iodine produced
  1. rinse burette with distilled water, then with standard solution of 0.1 mol/dm3 sodium thiosulfate
  2. fill burette with sodium thiosulfate, ensuring the jet is full
  3. place conical flask containing 25 cm3 solution on white tile
  4. record initial burette reading to nearest 0.05 cm3
  5. start rough titration, adding 1 cm3 of sodium thiosulfate at a time whilst continuously swirling the flask until colour changes to a pale straw colour, then add a few drops of starch solution
  6.continue titrating until colour goes from blue/black to colourless, record final reading to nearest 0.05 cm3. initial reading - final reading = rough titre
  7. fill a clean conical flask with 25 cm3 sodium thiosulfate and record new initial reading
  8. run sodium thiosulfate into conical flask until pale straw colour and add a few drops of starch
  9. continue adding sodium thiosulfate until 1 cm3 below rough titre, then add drop wise until colourless
  10. repeat steps 6-8 until you have 3 concordant results (that are within 0.1 cm3 of each other)