aspirin practical questions Flashcards
You are provided with a small sample of pure aspirin in a melting point tube. Describe briefly how you would determine an accurate value for the melting point of aspirin.
- heat melting point tube in oil bath
- slowly near melting point
Salicylic acid can be used to make aspirin. Before using a sample of salicylic acid to make aspirin, a student purified the acid by recrystallisation. The method for recrystallisation is outlined below.
Step 1: The sample is dissolved in a minimum volume of hot water.
Step 2: The solution is filtered hot.
Step 3: The filtrate is cooled in ice to form crystals.
Step 4: The crystals are collected by filtration, washed with cold water and left to dry.
Minimum volume and hot water 2 from:
- to obtain saturated solution/increase yield/enable crystallisation
filtered hot
- to remove insouluble impurities
cooled in ice
- to increase amount of crystals formed
washed with cold water
- to remove soluble impurities
give reasons for each of the following practical steps
- minimum quantity of hot water was used
- flask was cooled to room temperature before crystals were filtered off
- crystals were compressed in the funnel
- little cold water was poured through the crystals
minimum quantity of hot water was used
- to ensure hot solution is saturated/to enable crystals to form on cooling
flask was cooled to room temperature before crystals were filtered of
- lower yield of crystals when warm
crystals were compressed in the funnel
- air passes through sample not just round it
little cold water was poured through the crystals
- to remove/wash away soluble impurities
melting point of the sample in previous question found to be slightly lower than data book value.
suggest most likely impurity to have caused this low value and improvement to method so more accurate melting point can be obtained
impurity = water
compress sample between filter papers
A student as an impure sample of Ibuprofen that requires purification by recrystallization.
There are two solvents available for the purification; X and Y.
The sample dissolves in solvent X at room temperature
The sample only dissolves in solvent Y at 80oC.
Suggest which solvent should be used for the purification process and explain why.
Outline a method including steps that would ensure both soluble and insoluble impurities are removed. [7]
- solvent Y - To enable recrystalliasation and separation of impurities
method
Dissolve in minimum amount of hot solvent
To form a saturated solution
Filter hot Under reduced pressure
To remove insoluble impurities
Cool to crystalize Slowly with ice
To maximise crystals
Filter cold under reduced pressure
Wash with water to remove soluble impurities
Describe the method that should be used to purify an organic solid. [6]
- dissolve solid in minimum amount of hot solvent
- filter hot to remove soluble impurities
- cool to recrystallise
- filter under reduced pressure
- wash with cold solvent and dry
The paracetamol formed in this reaction is impure. It contains both soluble and insoluble impurities that must be removed by recrystallisation.
The properties of a suitable solvent for recrystallisation are shown.
* The paracetamol should dissolve when the solvent is hot but be almost insoluble when the solvent is cold.
* The impurities should either be insoluble in the solvent at all temperatures or soluble even in cold solvent.
* When a hot saturated solution of the paracetamol is cooled, as much product as possible should crystallise out, leaving soluble impurities in the solution.
A solvent has been suggested for this recrystallisation. It is a flammable liquid with a boiling point of 80 °C.
Outline how you would carry out an investigation to show that this solvent is suitable for the recrystallisation of the impure paracetamol. You should include brief practical details of how you would carry out your investigation.
You are not required to describe the full recrystallisation procedure.
Explain how you would check that a recrystallisation process had been effective at producing a pure sample of paracetamol.
- Place a small amount of pure paracetamol in a test-tube.
* Add small volume (0.5–1.0 cm3) of possible solvent
* Shake/stir.
* If paracetamol dissolves then solvent unsuitable.
* Heat (with suitable awareness of flammability and boiling point; e.g. hot − not boiling − water bath).
* If paracetamol does not dissolve = unsuitable.
* If paracetamol dissolves partially try adding more solvent.
* If/when completely dissolved place test-tube in ice-water bath.
* If crystals form = suitable.
* Effectiveness determined by measuring melting point.
* Purity indicated by melting point being sharp and close to 170 °C.
Suggest two reasons why, in an industrial situation, ethanoic anhydride would be preferred to ethanoyl chloride in the production of paracetamol.
* less exothermic reaction
* easier to control
* dangerous gas not evolved
* ethanoic anhydride is a cheaper or more easily recycled reagent.
Aspirin can be produced by reacting salicylic acid with ethanoic anhydride.
An incomplete method to determine the yield of aspirin is shown.
1. Add about 6 g of salicylic acid to a weighing boat.
2. Place the weighing boat on a 2 decimal place balance and record the mass.
3. Tip the salicylic acid into a 100 cm3 conical flask.
4. ______________________________________________
5. Add 10 cm3 of ethanoic anhydride to the conical flask and swirl.
6. Add 5 drops of concentrated phosphoric acid.
7. Warm the flask for 20 minutes.
8. Add ice-cold water to the reaction mixture and place the flask in an ice bath.
9. Filter off the crude aspirin from the mixture and leave it to dry.
10. Weigh the crude aspirin and calculate the yield
Describe the instruction that is missing from step 4 of the method.
Justify why this step is necessary.
M1 (Re)weigh the empty boat
M2 In order to calculate the (exact) mass of salicylic acid added to the reaction mixture
Suggest a suitable piece of apparatus to measure out the ethanoic anhydride in step 5.
10 cm3 measuring cylinder (if volume given – allow between 10 to 50 cm3)
Or a 10 cm3 pipette
Or burette / graduated pipette
Or 10 cm3 syringe
Identify a hazard of using concentrated phosphoric acid in step 6
Corrosive
Allow skin burn / permanent eye damage
Ignore irritant / toxic
A sample of the crude aspirin is kept to compare with the purified aspirin.
Describe one difference in appearance you would expect to see between these two solid samples.
Pure product will have (larger) crystals / needle-like crystals / lighter in colour
The pure aspirin is filtered under reduced pressure.
A small amount of cold ethanol is then poured through the Buchner funnel.
Explain the purpose of adding a small amount of cold ethanol.
To remove any soluble impurities
The crude aspirin can be purified by recrystallisation using hot ethanol
(boiling point = 78 °C) as the solvent.
Describe two important precautions when heating the mixture of ethanol and crude aspirin.
M1 (Ethanol is flammable so) use a water bath to heat / do not use a Bunsen burner
Must give practical step, not just state hazard
M2 Heat to temp below bp (so ethanol does not boil away)
Allow use min vol solvent
Suggest two ways in which the melting point of the crude aspirin collected in step 9 would differ from the melting point of pure aspirin.
M1 Value lower
M2 Range of values
For M2 allow mpt not sharp or a larger range of melting points
