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A2 Chemistry Unit 2 > Practicals > Flashcards

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1

Titrating iodine with sodium thiosulfate solution using starch and hence estimate oxidising agents by their reaction with excess acidified potassium iodide

1. Rinse pipette with sodium iodate(V) solution, using pipette filler, pipette 25cm^3 of standard potassium iodate(V) solution into conical flask
2. Add approximately 1.5g of potassium iodide (excess) and excess sulfuric acid
3. Rinse burette with standard sodium thiosulfate, fill burette w it
4. Add standard sodium thiosulfate from burette until solution is straw coloured, add 3 drops of starch indicator
5. Titration until blue-black to colourless
6. Repeat until results are concordant

2

Titrating acidified potassium manganate(VII) solution with reducing agents

1. Rinse pipette with solution of reducing agent and use pipette to transfer 25cm^3 to conical flask
2. Rinse burette w standard solution of potassium manganate(VII) and then fill burette
3. Add excess sulfuric acid using measuring cylinder to conical flask
4. Add potassium manganate(VII) solution from burette into conical flask and swirl until colourless to pink
5. Repeat until results are concordant

3

Determine purity of a group II metal oxide or carbonate by back titration

1. Weigh out mass of the solid
2. Add excess HCl
3. Add to 250 volumetric flask - make up w deionised water until meniscus on mark
4. Rinse pipette with solution from volumetric flask and pipette 25cm^3 to conical flask
5. 3 drops phenolphthalein
6. Rinse burette with alkali and fill
7. Add alkali until indicator changes colour
8. Repeat until concordant

4

Carrying out paper and thin-layer chromatography and measure the Rf values of the components and interpret the chromatogram

1. Base line in pencil close to bottom
2. Spot samples on line using capillary tube - dry - repeat till spot concentrated
3. Place paper in tank with shallow amount of solvent - cover with lid - allow solvent to run up over spots until solvent almost reaches top - mark solvent front - allow to dry
4. If substances are colourless then spray with locating agent eg ninhydrin
5. Measure dist travelled by the solvent front and each spot - calculate the Rf

5

Two-way chromatography

Chromatography method then
1. Rotate through 90°
2. Run in second solvent
3. Locating agent if colourless
4. Rf

6

Carrying out the reduction of acidified ammonium vanadate(V) with zinc and observing the sequence of colours

1. Dissolve some ammonium vanadate(V) in HCl in conical flask
2. Add two spatulas of zinc as reducing agent
3. Stopper with cotton wool, allow H to escape and slow entry of air
4. Observe sequence of colours

7

Determining the electrode potentials of a series of cells and predicting their values using standard electrode potentials

1. Set up a half cell - a metal dipping in 1M solution of its ions in a beaker
2. Attach via wires, and a voltmeter to a second half-cell using a different metal and a 1M solution of its ions
3. Use salt bridge to connect two beakers
4. Record the voltage

8

Preparing aspirin using salicylic acid and ethanoic anhydride

1. Place 1g of 2-hydroxybenzoic acid in a dry pear shaped flask and add 2cm^3 of ethanoic anhydride
2. Safely add 8 drops of concentrated phosphoric(V) acid
3. Heat under reflux for 30 mins
4. Add water to hydrolyse any unreacted ethanoic anhydride to ethanoic acid
5. Pour the mixture onto 400g of crushed ice in a beaker
6. Remove product by suction filtration
7. Recrystallise from water
8. Dry in low temp oven
9. Determine melting point

9

Using chromatography to compare the purity of laboratory-made aspirin with commercial tablets

1. Base line with pencil close to bottom and draw two pencil crosses on it
2. Place some laboratory-made aspirin solid on watch glass and dissolve in a few drops of solvent such as ethanol - spot solvent on pencil cross using capillary tube, dry and repeat until conc - repeat for commercial solid
3. Place in solvent, cover with lid, run up until near top - mark solvent front and allow to dry
4. Spots visible by placing in beaker containing iodine crystals
5. Rf