Methods of Practicals

Question 1

How would you carry out an experiment to find out the water of crystallisation in hydrated crystals?

Answer 1

• This method is suitable for any hydrated salt!!

METHOD

1. Weigh empty crucible
2. Add hydrated salt into weighed crucible. Weigh crucible and hydrated salt.
3. Using pipe-clay triangle, support crucible containing hydrated salt on tripod. Heat GENTLY for one minute and STRONGLY for a further three.
4. Leave crucible to COOL then weigh crucible and anhydrous salt.

Question 2

What would you do to find relative molecular mass?

Answer 2

• Ideal gas equation can be used to find RMM of volatile liquid

CONDITIONS
- Unknown compound needs to be a liquid at room temp. but has a boiling point below 100*C so it can vaporise.

METHOD

1. Add sample of volatile liquid to small syringe via needle, weigh small syringe.
2. Inject sample into gas syringe through self sealing rubber cap. Weigh small syringe to find mass of volatile liquid added to gas syringe.
3. Place gas syringe in boiling water bath at 100’C. Liquid vaporise producing a gas. Pressure is recorded. I

Question 3

How would you identify an unknown metal?

Answer 3

METHOD

1. Set up apparatus with a bung sealed flask with dilute HCl and an unknown metal in it. A gas syringe should be connect via an inlet tube.
2. Weigh a sample of metal and add to flask.
3. Using measuring cylinder, add 25.0 cm^3 1.0 moldm-3 HCl(aq) (an excess) to flask and quickly replace bung.
4. Measure maximum volume of gas in syringe.

Question 4

How do you prepare a standard solution?

Answer 4

• Known as the concentration of a know solution
• Volumetric flask is used to make standard solution very accurately.
  
  • Are used in either 100cm^3 +/- 0.20cm^3 or a 250 cm^3 +/-

METHOD

1. Solid is first weighted accurately
2. Solid is dissolved in a beaker using less distilled water than will need to be filled in a volumetric flask to the mark.
3. Solution is transferred to volumetric flask. Last traces are rinsed into flask with distilled water.
4. Flask is carefully filled to graduation line by adding distilled water a drop at time until bottom of meniscus lines up exactly at mark. Should be measured at eye level for accuracy and if solution goes over then should be done again as it will be too dilute.
5. Finally volumetric flask is slowly inverted several times to mix solution thoroughly. - if this isn’t done titration calculations are unlikely to be consistent. - you will also be able to see solution move through flask as more dense original solution moves through solution.

Question 5

How would a acid-base titration carried out?

Answer 5

CONDITIONS

• Solution of acid is titrated against solution of a metal hydroxide (base) by using a pipette or burette.
• Pipette is typically set to (10cm^3, +/-0.04cm^3) or (25cm^3, +/- 0.06cm^3)
• Burette is typically set to 50cm^3, +/- 0.10cm^3
• Burette has to be measure to nearest half division i.e. +/- 0.05cm^3 so every burette reading has to be to TWO D.P.
• If there’s an air bubble in burette, the air could be released during titration and can lead to error in titre.

METHOD

1. Add measured volume of metal hydroxide to conical flask using pipette.
2. Add acid to burette and record initial burette reading to nearest 0.05cm^3
3. Add few drops of indicator to metal hydroxide in conical flask.
4. Run solution in burette into solution in conical flask, swirl conical flask to mix two solutions. Colour change is seen at end point of titration.
5. Record final burette reading. Volume of solution added from burette is called ‘titre’.
6. Quick trial titration is carried out to find approximate titre.
7. Titration repeated accurately, adding solution dropwise as accurate titres are ‘concordant’ - i.e. within 0.10 cm^3 apart.

Question 6

What is a Quickfit apparatus composed of and what is it used for?

Answer 6

• To carry out an organic preparation.

1. Round-bottom or pear shaped flask.
2. Receiver
3. Screw-tap adaptor
4. Condenser
5. Still head

Question 7

How do you heat under reflux? Name apparatus used and safety measures that could be taken

Answer 7

• Round-bottom or pear-shaped flask.
• Condenser
• Rubber tubing
• Stand and clamp
• Heat source (a bunsen burner, gauze, tripod and heating mantle).
• water bath can be used if experiment is carried out below 100C.
• Using a heating mantle means there’s a lack of naked flame and provides extra level of safety should apparatus crack.
• Condensers are very fragile so should only be clamped loosely.
• NEVER put a stopper on top of condenser as it would become a closed system and pressure can build during heating then apparatus can explode.

Question 8

Can you explain the method of reflux heating?

Answer 8

METHOD

1. Flask is clamped by its neck and before condenser is fitted reaction mixture is added and anti-bumping granules so contents boil smoothly.
2. Finally apply thin layer of grease to ground-glass joint on condenser. Place condenser into flask and gently rotate it back and forth to provide good seal and make sure it comes apart after experiment is done.

Question 9

What is distillation used for, what apparatus is used and what method would you use?

Answer 9

• Can be conducted after reflux is complete, this is used to separate a pure liquid from its impurities.

APPARATUS:

• round-bottom/pear-shaped flask
• condenser
• rubber tubing
• heat source
• stand and clamp
• screw-cap adaptor
• still head
• thermometer

Method:

1. Flask is clamped by its head and still head is connected to flask.
   - Still head has a T-shape and is connected to fit screw cap adaptor and condenser. Grease joints so apparatus comes off easily after experiment.
2. Second clamp is placed around receiver adaptor at point where its attached to condenser.
3. Rubber tubing is used to connect condensers inlet tube with water to condense vapour that pass through tube.
4. A second flask is used to collect distillate so distillation apparatus is not airtight.
5. Original flask is heated and mixture will start to boil. Liquid with lowest boiling point is most volatile and will boil first.
6. Vapour moves out of flask into collecting flask via condensation.

Question 10

How do you purify organic compounds?

Answer 10

• You might notice two layers of substance in your collection flask, a way to identify which one is the organic coumpound or aqueous product is by adding water. If clear layer becomes bigger the coloured one is the organic compound.
• Once the compound has been identified, you need to use a separating funnel to separate both layers.

METHOD:

1. Ensure that tap of separating funnel is closed.
2. Pour mixture of liquids into separating funnel, place stopper in top of funnel and invert to mix contents.
3. Allow layers to settle.
4. Add water to see which layer increases in volume, this is the aqueous layer.
5. Place conical flask under separating funnel, remove stopper and open tap until whole of lower layer has left the funnel.
6. Place second flask under separating funnel to collect other layer

• It is possible during this process there could be acid impurities which can be removed by adding aqueous sodium carbonate and shaking mixture in funnel.
• This results in release of CO2 after acid reacts with NaCO3.
• Tap is to be opened with funnel upside down to release gas pressure.
• Aqueous sodium carbonate layer is removed and organic layer washed with water before running off both layers into separate flasks.

Question 11

What are the common inorganic salts used to dry organic liquids? Be specific what do they dry exactly!!!

Answer 11

• Anhydrous calcium choride - CaCl2 - used for drying hydrocarbons
• anhydrous calcium sulfate - CaSO4 - General drying
• anhydrous magnesium sulfate - MgSO4 - General drying

Question 12

How do you dry an organic liquid by removing water?

Answer 12

METHOD

1. Add organic liquid to a conical flask.
2. Using spatula, add some of drying agent to liquid and gently swirl to mix the liquids together.
3. Place stopper on flask to prevent evaporation of products. Leave for ten minutes.
4. If solid is clumped together than water is present, leave until solid represents a fine powder.
5. Decant liquid from solid into another flask. If liquid is dry it should be clear.

Question 13

How does redistillation work?

Answer 13

• A prepared sample may contain organic impurities so distillation apparatus is cleaned and set up so a second one can take place.
• This time only collect product of compound you are trying to make.
• The narrower boiling range = the purer the product.