Unit 4-Practical Skills And Techniques Flashcards
What is meant by the term concordant titre values?
Titre volumes added from burette should be ±0.1cm3 of each other
Name the piece of apparatus that uses the relationship between colour intensity of a solution and the concentration of coloured species present.
Colorimeter
Which technique uses the relationship between colour intensity of a solution and the concentration of coloured species present?
Colorimetry
What is the relationship between colour intensity of a solution and the concentration of coloured species present?
The higher the concentration of coloured species present, the higher the intensity of light absorbed.
Describe how a student can ensure the calibration curve is appropriate for determining the unknown concentration.
The calibration graph should cover the dilution range likely to be used in the determination of the concentration of the unknown solution.
If the solution of unknown concentration has an absorbance value outwith those used in the calibration graph, another calibration graph should be prepared using more appropriate concentrations of the standard solutions.
What is meant by heating under reflux?
Reflux involves heating the chemical reaction for a specific amount of time, while continually cooling the vapour produced back into liquid form, using a condenser.
Describe the apparatus used to heat a mixture under reflux.
The reaction mixture is placed in a round-bottomed flask with anti-bumping granules and the flask is fitted with a condenser. The flask is then heated using an appropriate source of heat (usually a heating mantle)
How would a control be used to validate a technique?
A determination would be carried out on a solution of known concentration.
Why would vacuum filtration be used instead of gravity filtration?
It is a faster way of separating the precipitate from the filtrate.
Why would a recrystallisation be used?
To purify a solid
How does recrystallisation work?
The impure compound is dissolved gently in the minimum volume of hot solvent then filtered to remove insoluble impurities. The filtrate is allowed to cool slowly to force crystallisation. The more soluble impurities are left behind in the solvent.
What must be considered when selecting a solvent for using in a recrystallisation?
Neither the compound nor the impurities should react with the solvent. Other factors to consider will be the solubility of the compound and the impurities in the solvent, and the boiling point of the solvent (if the boiling point is too low, it will not be possible to heat the solvent to dissolve the impurities as it will evaporate off too easily).
How is a solvent extraction carried out?
Two solvents which are immiscible, for example an organic solvent such as diethyl ether and water are mixed.
The organic solvent can be used to extract an organic compound from an aqueous solution leaving water soluble impurities behind.
This is carried out in a separating funnel.
What is a potential risk when using a separating funnel?
How is this risk lowered?
Pressure build up.
Opening the stopper routinely while mixing with the top of the funnel point away from the body.
What is the purpose of Thin-layer chromatography?
To assess product purity
