Practical Skills Flashcards
Measuring mass
Weighing mass by differ pace is more accurate than directly measuring on scale. If the boat measures more than 0g after transfer then wash the boat with distilled water
Measuring volume of liquid
In titrations a dropping pipette is not accurate so you would use a volumetric pipette as high precision
Reducing percentage uncertainty in titration
- Increasing volume of titred
- Decreasing the concentration of what is being titred
Measuring the volume of gas
Gas syringe or trough should be used to measure the volume of gas
Measuring temperature in enthalpy change practical
This is done by minimising heat loss or heat gain from the surroundings. a polystyrene cup and a lid is used to insulate the reaction mixture. The stirrer is used to ensure thermal energy is spread evenly. The bottom of the thermometer must be in the reaction mixture.
Measuring pH
To reduce uncertainty, you must stir the solution and move the pH probe before reading to ensure the concentration of H plus ions is the same in the solution. You should also construct a pH calibration curve as the pH meter will have systematic error due to residue off other solutions
Measuring colour- transition metals
Use a colorimeter
1. Add an appropriate ligand to intensify the colour
2. Make up Solutions with known concentration and measure the absorbent
Plot calibration curve of being concentration and why axis is being transmission
3. Measure the transmission of unknown concentration and read of calibration curve to find the measured concentration also draw a line of best fit
Making up standard solution Method.
- Use mass by difference to weigh the solid and
- Add distilled water and stir with the glass rod until all the solid has been to
- Transfer to a volumetric flask with washing
- Make up 250 cm³ mark with the distilled
- Shake flask
Titration errors
- Remove the funnel.= so extra droplets don’t fall into the titre and predict a larger titre.
- Fill the Air bubble so does that lead to a larger than expected titre value
- Reduce percentage uncertainty by increasing volume of titre and decreasing concentration of what is being
- Dealing with acid and alkali which are corrosive so make sure to wear eye protection and gloves
Preparation of cyclohexene by the dehydration and distillation of cyclohexanol method
- Pour 20cm3 of cyclohexanol into a pear shaped flask that has been weighed by mass by difference and redid the mass of cyclohexanol.
- Use a plastic graduated poppet to add concentrated phosphoric acid which acts as a catalyst to the flask and shake continuously
- Add anti bumping granulated to prevent formation of large vigorous bubbles and heat gently using a water bath or electric heater as it’s flammable.
- Pour into distillate using a separating funnel and add sodium chloride solution which acts as a drying agent to allow two layers to separate.
- Transfer the upper layer which contains cyclohexene into a small conical flask
- Add a few lumps of calcium chloride to remove water
- The can the liquid into a clean dry sample container that has been weighed and calculate the dry mass of this and determine the percentage yield of your product
7.Test this using brown water to confirm that it contains an alkene
How to calculate rf value
Distance travelled by component / distance travelled by solvent
How to carry out TLC?
- Add a solvent t o a jar and seal to create a saturated environment
- Draw a pencil line on the TLC plate and spot samples along this using a capillary tube
- Add TLC plate to ja. Make sure the solvent is below the pencil line
- The solvent rises up the TLC plate bringing the component of the sample with it
- Draw a pencil line where the Lovett finishes and don’t allow it the travel to the top of the plate. Allow it to dry
Why must the line be in pencil
If drawn in ink it will dissolve in the solvent and run up the plate contaminating the plate
Why should the solvent be below the pencil line?
If above the pencil line the solve it will dissolve all the samples and they will not run up the plate
What happens if your sample is too concentrated?
Spots will overlap
How do you view the spot on the TLC plate?
- Some spots may be visible or may be visible using UV light.
- Sometimes the plate is sprayed with a chemical to show invisible spots.
What are the main steps in producing a pure organic solvent?
- Synthesis of compound by reflux or distillation.
- Filtration using a vacuum filtration.
- Purification such as recrystallisation
Why is heating under reflux used?
- Allows heating for a long period of time.
- Ensures even heating.
- Prevent products or reactants from escaping
What does a diagram of a filtration under reduced pressure look like?
- Use a Büchner funnel with a Buchner flask connected by rubber tubing to the vacuum store
- Funnel contains a layer of funnel paper
- Pot the substance onto the funnel paper and liquid will be sucked via vacuum filtration
- Solid will remain in the paper
How do you purify a solid compound?
By recrystallisation
1. Add minimum amount of hot solvent to impure sample and filter hot until it has dissolved
2. Allow to cool and crystals should form in ice
3. Crystals are collected by filtration and washed with cold water and left to dry.
How would you determine the melting point of a substance and why can this information be useful?
- Place a small sample of the solid in a capillary tube.
- Melt using melting
Apparatus available and measure the temperature with a thermometer - a pure sample will melt at a single temperature but an impure sample will melt over a range of temperatures which is lower.
- Record the starting and ending point of the melting when the first crystal can be seen to melt and when the last crystal becomes a liquid respectively.
- You can then compare the melting point to known values to identify the substance
Explain the purpose of minimum volume
To get a high yield of aspirin and to enable crystallisation
Explain the purpose of hot water
salicylic acid doesn’t crystallise during filtration
Explain the purpose of cooled in ice
Increase the amount of crystal formed
Explain the purpose of washed with cold water
To remove soluble impurities
How do impurities affect melting point?
Impurities distort the bonds and weakens them
This lowers the melting point of the bonds as we intermolecular forces
Therefore, less energy required to overcome these forces
The melting point will be over a range of different temperatures
