Practical Skills Flashcards
What is recrystallisation used for?
Purifying an impure substance
Describe the method of a student aiming to purify a substance (8)
Recrystallisation
1) Place boiling tube and glass filter in the oven so it’s hot
2) Set up a water bath in 250cm3 beaker and start heating the water
3) Place boiling tube 2/3 full of deionised water in water bath
4) Weigh out 1g of substance into another boiling tube and place in the water bath
5) Once water is hot, add water from boiling tube of deionised water into impure substance. Dissolve substance in a minimum volume of hot water
6) Collect hot glassware from oven, and place filter paper in the funnel. As quickly as possible, pass solution through the filter
7) Leave filtrate to cool and allow substance to crystallise
8) Once cooled and crystallised, filter off crystals under reduced pressure. Wash off with cold deionised water, and dry crystals in a warm oven
How are mixtures separated through chromatography?
Mixtures separated based on the differences of solubility in the mobile phase, and the differences of affinity to the stationary phase.
Affinity in chromatography
The more attracted a substance is to the stationary phase (stronger IM forces of attraction) the less it moves up the stationary phase
Solubility in chromatography
The more soluble a substance is in the mobile phase, the further up it moves up the stationary phase
What is solvation used for and how does it work?
Used to separate 2 solid solutes. One of the solutes is more soluble in the solvent since it forms IM forces of attraction with the solvent. The other solute doesn’t.
Explain the choice of solvent in recrystallisation
Solute should be soluble in hot solvent but insoluble in the cold solvent - so it can crystal out
How do you find the melting point of a sample (2)
1) Place sample in a capillary tube and seal at one end.
2) Place capillary in the melting point apparatus and observe the melting point
Explain the method of determining the water of crystallisation of Magnesium Sulphate (7) MgSO4.xH20
1) Find mass of an empty boiling tube. Record this and other masses in a suitable table
2) Place about 2cm depth of hydrated MgSO4 in the boiling tube and find the mass
3) Heat the tube to constant mass. If steam is still being given off from the tube, there is still water of crystallisation to be lost
4) Calculate moles of MgSO4 at the end.
5) Use answer to (4) to calculate moles of MgSO4.xH20 at the start
6) Use answer to (5) and mass of MgSO4.xH20 at the start to find its relative formula mass
7) Work out value of x
Explain the method of preparing a standard solution (6)
1) Measure mass of solute using a mass balance
2) Dissolve solute in a small volume of distilled water
3) Transfer to volumetric flask
4) Rinse beaker, stirring rod and funnel with distilled water (pour into flask)
5) Carefully make up to mark with distilled water
6) Add stopper and invert to mix thoroughly
Explain the method of determining the enthalpy change of a reaction (6) - Displacement - zinc powder to copper (II) sulphate solution
1) Weigh out 3.00g +-0.01g of zinc
2) Place a clean, dry polystyrene into outer plastic cup to increase insulation and stability
3) Pipette 25.00cm3 of 0.500 moldm-3 coper sulphate solution into cup
4) Place lid on the cup and clamp thermometer through hole in lid. Stir and record temp. every 30 seconds for 2.5 minutes
5) At 3 minutes, add zinc to cup
6) Continue stirring and record temp. every 30 seconds until 10 mins (or temp has fallen) - Record in a suitable table
Graph for displacement reactions to find max temp
Extrapolate back to 3.0 minutes
Graph for neutralisation reactions to find max temp
Work out intersection
What is the formula for percentage uncertainty?
(uncertainty of measurement)/measurement) x 100
What is the formula for percentage error?
((experimental value - accepted value)/accepted value) x 100
