Last Minute Cram - Researching Chem Flashcards
weighing by difference
most accurate method of measuring mass of products
a clean, dry weighing boat is first weighed empty and then the sample added to it
mass of weighing boat+contents is recorded
sample transferred and gentle tappig on the base to ensure sample is all transferred
weighing boat weighed again and difference found between final and initial.
desiccator
a closed vessel that contains a desiccant (drying agent) in its base.
main desiccant is self-indicating silica gel - blue when dry and pink when it absorbs moisture
vacuum filtration
technique used for seperating a solid product from a liquid
carried out under reduced pressure and is faster than other methods of filtration
mixture is poured through a filter paper into a Buchner funnel
solid is trapped by the filter and liquid is drawn through the funnel into a flask below by a vacuum
refluxing
technique used to apply heat energy to a chemical reaction mixture over an extended period of time
heated vigorously over the course of a chemical reaction; any vapours given off are immediatelyreturned to the reaction vessel as liquids when they reach the condenser.
distillation
used to purify a compound from a mixture of liquids
process of heating and cooling liquids to separate them
different b.p. to separate more volatile from less volatile
difference of 15 degrees needed
solvent extraction
purify a compound by using its different solubility between two solvents
second solvent added that is immiscible in water
separating funnel used
recrystallisation
used to purify solids by removing by-products
- dissolve impure crystals in min. volume of hot solvent
- filter the hot solution to remove insoluble impurities
- allow solution to cool and crystalise
- filter off crystals using vacuum filtration and wash with a cold solvent
- allow crystals to dry in the oven and then cool in a desiccator
mixed melting-point determination
mixing a small quantity of the product with some of the pure compound (50% of each)
if m.p. sharp and close to expected value - pure
m.p. much lower than expected and melting range much broader -impure
percentage yield
will always be less than 100% due to:
losses when products are being transferred, product remains in solution, side reactions, impure reactants, position of equilibrium…
what is a standard solution
a solution of accurately known concentration
when can a standard solution be prepared directly from a solute?
if the solute is a primary standard
primary standard
high purity stable in air and solution readily soluble in solvent (usually water) solubility high enough large gfm
why is sodium hydroxide not suitable as a primary standard?
relatively low gfm
unstable as solid and solution
examples of primary standards
sodium carbonate
oxalic acid
silver nitrate
potassium dichromate
preparing a standard solution
mass of solute weighed
dissolved in solvent in a beaker
transferred into standard flask along with washings of the beaker
solution made up to the mark (dropwise when close)
stoppered and inverted to mix
