4.1 researching chemistry Flashcards
what is stoichiometry
the study of mole relationships involved in chemical reactions
what is percentage by mass
the mass of solute made up to 100cm3 of solution
what is percentage by volume
the number of cm3 of solute made up to 100cm3 of solution
what is gravimetric analysis used to determine
the mass of an element or compound in a substance
what occurs in precipitation conversion
this is when the substance undergoes a precipitation reaction. the precipitate is separated from the filtrate and the filtrate is tested to ensure the reaction has gone to completion. the precipitate is washed, dried to constant mass and then weighed.
what occurs in volatilisation conversion
the substance is heated and any volatile products are evaporated. the substance is heated to constant mass and the final mass is recorded
what is a standard solution
a solution of accurately known concentration
what must a primary standard be
- available in a high state of purity
- be stable when solid and in solution
- be soluble
- have a reasonably high gfm
what are examples of primary standards
- sodium carbonate
- hydrated oxalic acid
- potassium hydrogen phthalate
- silver nitrate
- potassium iodate
- potassium dichromate
why is sodium hydroxide not a primary standard
it has a low gfm, is unstable as a solid and as a solution. it must be standardised before being used in a volumetric analysis
what can EDTA be used to determine
the concentration of metal ions in solution
describe a back titration
- used to find the number of moles of a substance by reacting it with an excess volume of a reactant of known concentration
- the resulting mixture is then titrated to work out the number of moles of the reactant in excess
- from the initial number of moles of that reactant the number of moles used in the reaction can be determined.
- the initial number of moles of the substance being analysed can then be calculated.
when is a back titration useful
when trying to wok out the quantity of substance in a solid with a low solubility
what does a colorimeter use the relationship between
colour intensity of a solution and the concentration of the coloured species present
what is a colorimeter used for
measuring absorbance of light of a series of standard solutions, and this data can be used to plot a calibration graph
what is distillation used for
identifiction and purification of organic compounds
what does heating under reflux allow
heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping
what does heating under reflux consist of
placing the reaction mixture in a round bottomed flask with anti bumping granules and fitted with a condensor. the flask is then heated using an appropriate source of heat
what is vaccuum filtration
this technique involves carrying out filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate
what does recrystallisation involve
- dissolving an impure solid gently in a minimum volume of hot solvent
- hot filtration of the resulting mixture to remove any insoluble impurities
- cooling the filtrate slowly to allow crystals of the pure compound to form, leaving soluble impurities dissolved in the solvent
- filtering, washing, and drying the pure crystals
how is the solvent for recrystallisation chosen
so that the compound being purified is completely soluble at high temperatures and only sparingly soluble at lower temperatures
what does solvent extraction involve
isolating a solute from a liquid mixture or a solution by extraction using an immisible solvent in which the solute is soluble
describe the steps to a solvent extraction
- two immiscible solvents form two layers in the separating funnel
- the solute dissolves in both solvents and an equilibrium establishes between the two layers
- the ratio of solute dissolved in each layer is determined by the equilibrium constant
- the lower layer is run off into a container and the upper layer is poured into a second container
- this process is repeated to maximise the quantity of solute extracted
how can you maximise the quantity of solute extracted
using a number of extractions using smaller volumes of solvent rater than a single extraction using a large volume of solvent
what can the determination of the melting point of a compound indicate
the purity of the compound
what is chromatography
a technique used to separate the components present within a mixture. it separates substances by making use of difference in their polarity of molecular size
what does the distance travelled in chromatography depend on
how soluble the compounds are in the chosen solvent and how well they adhere to the plate. a developing agent or ultraviolet light is normally required to visualize the spots on the chromatogram
how does thin layer chromatography assess the purity of substances
a pure substance when spotted and developed on a TLC plate should appear as a single spot. the presence of more than one spot shows that impurities are present
