Practical questions unit 3 Flashcards
Primary standard characteristics
High purity
Stable in air and solution
Soluble in solvent such as water
Large GFM/RFM
Reason for high purity
To ensure the mass weighed out is the mass of the required chemical - meaning there is minimised error on the actual mass of the required chemical.
Reason for stability in air and solution
Some of the required chemical will react with the air or solution if unstable in these conditions meaning there will be less of the chemical than measured out.
Reason for solubility in water or other solvent
To allow for high masses of solute to dissolve - which will allow high concentration solutions to be prepared.
Reason for large gfm
To minimise error and uncertainty of the number of moles of a substance wieghed out.
What are primary standards
Chemicals which can be directly used to produce standard solutions.
Producing standard solution from primary standard
Ensure the primary standard sample is dry and has absorbed no water.
Weigh by difference mass of primary standard accurately approximately.
Dissolve the primary standard in 50cm3 of deionised water in clean beaker.
Pour into standard flask using filter funnel.
Wash out any excess of primary standard from beaker and filter funnel.
Make up the standard solution to the graduation mark with the bottom of the meniscus on the line.
Stopper and invert.
Titration procedure
Produce a standard solution.
Rinse out burette with solution including the tip by draining the solution out.
Then fill the burete with solution using a filter funnel at eye height.
Remove filter funnel.
Rinse out a pipette with the analyte.
Transfer fixed volume into conical flask with pipette.
Carry out a rough titration by adding the solution quickly to the analyte. Use a white piece of paper to help determine the endpoint.
Repeat by adding the solution to the analyte slower carrying out proper titrations until concordant results are found.
Heating to constant mass process
Heat the crucible and lid under Bunsen flame for 10 minutes
Allow crucible to cool and transfer into dessicator.
Weigh out mass of substance accurately approximately into crucible.
Heat under Bunsen flame gently for 2 minutes
Then heat for 10-15 minutes under blue flame.
Then place in dessicator and weigh once cooled.
Reheat for 5 minutes under roaring blue flame until masses between 0.01 g are recorded.
Back titration process
Weigh the mass of the solid sample by difference accurately approximately.
Prepare a standard solution with known concentration to react with the solid sample.
React the solid sample with the first standard solution ensuring that the standard solution is in excess.
Prepare a standard solution to react with standard solution 1.
Titrate the standard solution1 using standard solution 2.
Use N=CV
Mole ration
N= CV
Mole subtraction
Mole ratio M= NxGFM
Colorimeter containers name
Cuvette
Colourimetry process potassium permanganate calibration graph
Make up standard solution of potassium permanganate.
Rinse burette out with the standard solution and then measure out 0,2,4,6,8,10,12, and 14 cm3 of potassium permanganate into 50cm3 standard flasks.
Make the solutions up to the graduation mark with deionised water and ensure the bottom of the meniscus is on the line.
Stopper and invert all standard flasks.
Pour a sample of each solution into a cuvette and fill one cuvette with deionised water for a reference reading.
Ensure correct filter is used for the experiment.
Use the Colourimeter to find the absorbance of each solution and plot a calibration graph.
Colorimetry process changing steel to permanganate
Swirl paper clip in propanone to get rid of grease on the surface.
Cut off section of the paper clip and weigh mass by difference using a beaker in a balance.
Add nitric acid to the beaker and cover with a clock glass
Heat gently in fume cupboard until all steel has reacted
Let cool, add anti bumping granules and then boil off brown fumes.
Colourimetry calculation
The concentration is directly correlated to the absorbance.
Use N=CV
Use sample factor to standard flask.
Use M=nxgfm
Process of organic chemistry techniques
Preparation - reaction takes place
Isolation - product removed from the mixture
Purification - purified
Identification - confirm product and calculate percentage yield.
Heating under reflux process
Weigh by difference the limiting factor in the reaction in round bottomed flask.
Add the excess reactant to the round bottomed flask along with anti bumping granules.
Assemble glassware by attaching the condenser to the top of the round bottomed flask.
Use heating mantle to heat the mixture, then allow to cool.
Vacuum filtration process
Put filter paper into the Buchner funnel.
Assemble apparatus by placing Buchner funnel into flask and pipe into the water pump.
Turn pump on and pour mixture into the funnel in small samples.
Then wash out the beaker or flask with deionised water until all of the mixture is in the funnel.
Wait until completely filtered.
When would you carry out distillation
When the desired product you want to isolate is a liquid and more volatile than all other products in the mixture.
Distillation
Assemble glassware by attaching reaction flask to a still head and the still head to the condenser sideways, the condenser is then put into a receiving adapter and into a receiving flask. A screw thread adapter is then placed on top of the still head and a thermometer on top.
Add anti bumping granules.
Switch on heating mantle and heat to the desired products boiling point.
(Higher bond of the desired product but lower than all other products.)
Recrystallisation procedure
Use a solvent which the solid is insoluble in at low temperatures but soluble at high temperatures.
Pour minimum volume of solvent necessary for the solid to dissolve
Heat gently on an hot plate until solute dissolves .
Allow the solution to cool and for crystals to form in the solution.
Use vacuum filtration to filter off solution containing impurities.
How do you remove water from a liquid or solid product
Place into a desiccator with a drying agent.
When do you use fractional distillation
If the impurities are more volatile than the product.
Difference between fractional distillation and distillation setup
The fractional distillation apparatus has a fractionating column.
Melting point range definition
The temperature where the product begins to melt and the temperature where it completely liquifies.
If pure it will have a mp range of 1 degrees Celsius.
Impurities affect on mp
Pure products have a sharp and definite mp
Impurities will increase the range of the mp since different substances within the mixture will have different melting points.
Impurities will decrease the mp since the molecules will be less closely packed together which will decrease the strength of the intermolecular forces holding the molecules together.
Melting point determination procedure
Crush sample into powder.
Transfer into capillary tube
Put capillary tube into oven with thermometer.
Raise within 25 degrees Celsius of mp
Then increase temperature by 2 degrees every minute
Record temp when solid begins to melt and when solid has completely melted
Reason for mixed melting point determination
Mixing a pure sample with your sample in equal proportions will make the mp sharper and more definite mp.
How do you identify the product if it’s a liquid
Use boiling point instead of melting point.
Thin layer chromatography procedure
Use a thin layer plate and draw a pencil line 1cm from the bottom
Fill container with high volatility solvent ensuring that the solvent remains below where the TL plate pencil mark would be drawn.
Spot the solution onto the TLC plate using a capillary tube - use a control pure sample and your sample as well.
Transfer the TL plate into the container and place clock glass on top to prevent evaporation.
Measure Rf
Rf value
Distanced traveled by substance
/ distance travelled by solvent.
Percentage yield
(Actual yield/ theoretical yield) x 100
Clock glass
A piece of glass used to put chemicals on or cover the top of a beaker or flask.
Heating mantle
A heating apparatus with a circular bottom to heat round bottomed flasks.
Dessicator
A container which contains a drying agent to remove water impurities.
Condenser
A piece of chemistry glassware attached to a water pump at the top which allows for heating under reflux and distillation
Colourimeter
A machine which has filter and directs white light at a solution sample to determine absorbance.
Büchner funnel + flask
Apparatus used in vacuum filtration to remove impurities using a water pump.
