practical techniques Flashcards
rules for reading burettes and measuring cylinders
- read to bottom of the meniscus
- read to half the unit marked
rule for reading thermometers
read to half the smallest unit marked
method for weighing out substance using a weigh boat
- weigh boat + contents
- tip out contents
- weigh boat again
plotting graphs
plot the independent variable (the one you change) on the x axis, and the dependent variable on the y axis
how to find the gradient
change in y divided by the change in x
notes on colour observations
- if there is a colour change, state the starting colour and the finishing colour
- no colour = you need to say colourless, not clear
significant figures in calculations
- in the intermediate answers, keep at least one more significant figure than you will need in the end
- in the end answer, use the same sig figs as the smallest number of significant figures in the data given
data that needs to be given in a final answer
- sign (plus or minus)
- units
- significant figures
purpose of a titration
to find the volumes of solutions that react
calculating concentrations in titrations
- write a chemical equation for the reaction (or look at given equation)
- work out the number of moles of one of the reagents
- use the chemical equation to work out the number of moles of the reagent that reacts with reagent one
- calculate concentration of reagent 2
working out uncertainty
uncertainty can be taken to be the same size as the smallest unit that can be measured on the instrument (i.e. half the smallest division marked on the instrument)
working out percentage uncertainty
( uncertainty / reading ) x 100
rule when multiplying and dividing quantities with uncertainties
add the percentage uncertainties
rule when adding and subtracting quantities with uncertainties
add the actual uncertainties, not the percentage uncertainties
what is precision in chemistry
the repeatability of results
calculating percentage error
( difference between experimental result and theory results / theory result) x 100
identifying a systematic error
if the experiment is not accurate, the deviation from the theory value must be due to a systematic error
types of systematic error
- operator error
- apparatus error
- method error
potential causes of systematic error and their ways of minimising
- heat loss from the apparatus ~ cover the top of the beaker
- heat loss to the beaker ~ use metal calorimeter
- evaporation of fuel ~ place cap on burner
method to make up solutions
- calculate the mass of substance you need to weigh out
- dissolve substance in 100cm3 of distilled water
- pour solution into volumetric flask
- rinse beaker and funnel with distilled water into volumetric flask
- add distilled water to flask until it reaches level line
- stopper the flask and invert 10 times for a homogenous solution
method for dilution solutions
- estimate volume of diluted solution needed
- calculate volume of undiluted solution you will need using (new volume x new concentration = old volume x old concentration)
- measure out the volume of undiluted solution in the most precise way you can
- run this volume into volumetric flask
- make flask up to mark with distilled water and invert 10 times for a homogenous solution
apparatus for titrations
- conical flasks
- beakers
- burettes
- pipettes
- pipette filler
procedure for titrations
- set up apparatus
- add reagent one in burette, add reagent two into conical flask on white tile below the burette
- add 5 drops of indicator (if needed) into conical flask
- record initial burette reading (miniscus at eye level)
- run solution from burette into titration flask until indicator just changes colour (add drop by drop at end point)
- record burette reading, calculate titre
- rinse out titration flask with distilled water and repeat to until you have three concordant results
- calculate an average titre
what are concordant results
results that are within 0.1cm3 of each other
what is heating under reflux
using a water condenser mounted vertically above a heated flask, top of the flask is not stoppered
uses of heating under reflux
useful for slow reactions with volatile reactants and products that would otherwise escape
also useful because many organic liquids are flammable
use of distillation
to remove one liquid from a mixture of liquids
method for purifying a liquid
- shake liquid with various substances in a separating funnel
- if any gases given off, the funnel must be held upside down after shaking and pressure released through the tap
- allow funnel to stand until layers have separated, run off a layer and collect it
- if an organic layer has water ass the main impurity, it can be dried by leaving it to stand over anhydrous sodium sulfate in a flask with a stopper, then filter through a mineral wool plug
- distill to remove any remaining impurities
method for purifying a solid
vacuum filtration
- a moistened filter paper of the correct size is placed in buchner funnel
vacuum pump (tap) is turned on and a suspension of the solid required is poured in
when liquid is drained, solid is left on filter paper, which can then be recrystallised
method for recrystallisation
- set up a hot water bath, containing two boiling tubes one containing solvent (e.g. ethanol) and the other containing the solid
- add hot solvent to solid in small proportions and stir the mixture each time. stop adding immediately after a clear solution is formed
- leave solution to cool and recrystallise
- use vacuum filtration, filter off the solid, then wash it with a little cold solvent and allow it to dry
calculating percentage yield
(actual mass of product / theoretical mass) x 100
- based off number of mole of limiting reactant
use of colorimetry
used to measure the concentration of a coloured substance in a solution
steps for colorimetry
- select filter (complementary colour of the tested solution)
- zero colorimeter with a cuvette of distilled water
- make up several standard solutions of the coloured substance
- measure absorbance for each of these solutions
- plot a graph of absorbance against concentration
- plot line of best fit to get calibration curve
- find absorbance for solution of unknown concentration, read off the concentration of of the unknown from calibration curve
thin layer chromatography procedure
- use a pencil, draw a base line on a thin layer plate, making it above the level of the solvent
- use capillary tube, place spots of known and unknown substances on baseline
- place the plate in a solvent in a beaker
- cover beaker with clingfilm
- when solvent is near top of the plate, remove plate and mark solvent level
- allow plate to dry and then spray with ninhydrin solution to see spots
- calculate Rf value and compare substances
calculating Rf value
Rf = distance moved by spot / distance moved by solvent front
