chemistry practical techniques Flashcards
(20 cards)
how to measure and record the mass
-use a top pan balance (or analytical balance)
-analytical balance gives a very accurate mass to 4dp and top pan balance gives to 2 dp
-ensure the balance is always zero before use
-and ensure weigh by difference is used for weighing boat
what is weigh by difference
-weigh mass of weighing boat
-mass of weighing boat and solid used
-add solid to reaction
-weigh mass of weighing boat after and calculate how much solid has been transferred
how to measure and record the time
-use a digital stopwatch
-measure the time taken for the reaction
-stop at precisely the correct moment
how to measure and record the volume of liquids
-pipette for fixed, very accurate volumes (to fill a pipette filler can be used)
-Burette for variable but precise volumes (ensure the readings are from the bottom of the meniscus)
-volumetric flask for making up standard solutions
-measuring cylinder for approximate volumed
-for all ensure rinsed with distilled water before solution is used
how to measure and record the volume of gases
method 1:
-gas syringe for precise volumes
-ensure it is leak proof
method 2:
-use an upside-down measuring cylinder in a water trough which is used for less precise collection by water displacement
-ensure the measuring cylinder is filled with water and is inverted properly
how to measure and record the temperature
-digital thermometer
-glass thermometer (can be alcohol or mercury)
-make sure the solution is stirred before using
-ensure it is properly immersed in solution
when should a water bath or electric heater be used instead of a bunsen
if there are flammable substances present
how to use a water bath
-immerse the beaker into the water bath
-heat using a bunsen burner but ensure not overheating
-use a thermometer to monitor heat and heat or add cold water accordingly
(can be maintained using hot water such as form a kettle and cold water ensure that question is read)
important things when using an electric heater
-can place above the heater using a tripod stand with gauze mat
-thermometer should be in solution but not touching the sides of bottom of any glassware
-don’t fill too full as need to leave room for expansion
-set initially to low temperature and gradually increase if needed
-use natibumpting granules (or boiling stones)
how to use a sand bath for heating
-place metal container in electric heater and fill with sand
-place the flask in sand ensuring it is stable and covers the bottom and sides but does not get in the solution
-gradually increase temp if needed
-monitor with thermometer which is in solution but does not touch glassware
-use anti bumping granules
why is a water bath used
-uniform heating
-prevents overheating as for gentle or controlled heating
-safe for flammable or sensitive chemicals such as volatile substances
-temp is naturally limited to 100 degrees
when is electric heater used
-temperatures can be controlled so can be used for high temperatures
-for flammable substances
when is a sand bath used for heating
-even heat distribution and can heat slowly and gently
-for flammable substances
-can reach high temperatures such as over 100 degrees
-reduces risk of glass cracking
how to use melting point apparatus
-electronic melting point apparatus
-capillary tube strapped to a thermometer which is immersed in heating oil
-salt is placed into the capillary tube and is heated gently until all is melted
how are impurities found on melting point
-pure sample melting point will be very sharp
-impurities cause melting over a range of temperatures which are lower than the melting point
how to do recrystallisation
-dissolve the crude product in the minimum volume of hot solvent
-filter to remove insoluble impurities
-cool and allow to recrystallise
-filter under reduced pressure
-wash with cold solvent and dry
how to purify an organic liquid product
separating funnel
how to purify an organic solid product
recrystallisation
how to use a separating funnel
-pour into a separating funnel and add stopper
-invert and open stopcock to release the pressure. ensure this is repeated at least 15-20 times
-the two layers (aqueous layer and organic layer have seperated)
-open stopcock and allow pour the organic layer into beaker
-add some solid sodium sulfate (anhydrous) as is a drying agent to removes water
how is TLC done (can be adapted as similar for paper chromatography)
-on TLC plate draw the baseline in pencil
-spot of pure reference compound and a spot to be analysed and allow to air dry (the reference compounds allow identification of mixture of compounds in the sample)
-place TLC plate in beaker with solvent ensuring the solvent is below the baseline (pencil line)
-allow solvent to travel up the TLC plate but ensure it is removed from the solvent before it reaches the top of the paper
-draw line in pencil where solvent has reached as this indicates solvent front
-to locate spots a UV light, ninhydrin or a iodine vapour may need to be used
-calculated Rf value as distance traveled by the substance/ distance traveled by the solvent
