Researching Chemistry Flashcards
Stoichiometry
the study of mole relationships involved in chemical reactions.
Weighing by difference
Weighing a substance in one container then transfer it to a different container. Weigh the substance in a container then subtract the mass of the substance from the mass of the container and the substance
Weighing accurately approximately
Weigh as close as possible to the specified mass but if mass is different incorporate that into an calculations etc.
Heating to a constant mass
—heating a substance
— allowing to cool in a desiccator to prevent absorption of water
— weighing
—repeating steps of heating cooling and weighing until no further changes in mass are observed.
Precipitation conversion
the substance undergoes a precipitation reaction. The precipitate is separated from the filtrate and the filtrate tested to ensure the reaction has gone to completion. The precipitate is washed, dried to constant mass and then weighed.
Volatilization conversion
the substance is heated and any volatile products (often water) are evaporated. The substance is heated to constant mass and the final mass recorded.
How to make a standard solution
♦ weighing a primary standard accurately
♦ dissolving in a small volume of solvent (usually deionised or
distilled water) in a beaker
♦ transferring the solution and rinsings into a volumetric flask
♦ making up to the graduation mark with solvent
♦ stoppering and inverting
A primary standard must
be available in a high state of purity
be stable when solid and in solution
be soluble
have a reasonably high GFM
Examples of primary standards
sodium carbonate, Na2CO3
hydrated oxalic acid, H2C2O4·2H2O
potassium hydrogen phthalate, KH(C8H4O4 )
silver nitrate, AgNO3
potassium iodate, KIO3
potassium dichromate, K2Cr2O7
Why is sodium hydroxide not a primary standard
it has a relatively low GFM, is unstable as a solid (absorbs moisture) and unstable as a solution.
Heating under reflux
Heating under reflux allows heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping.
When carrying out heating 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.
Vacuum Filtration
Vacuum filtration involves carrying out a filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate. A Büchner, Hirsch or sintered glass funnel can be used during vacuum filtration.
Recrystallization to purify an impure solid
dissolving an impure solid gently in a minimum volume of a 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
Solvent extraction
Solvent extraction involves isolating a solute from a liquid mixture or solution by extraction using an immiscible solvent in which the solute is soluble.
When carrying out a solvent extraction, the 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, K . 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.
What solvent should be used for solvent extraction
immiscible with the liquid mixture or solution (usually water)
one in which the solute is more soluble in than the liquid mixture or solution (usually water)
volatile to allow the solute to be obtained by evaporation of the solvent
unreactive with the solute
