EMPA Flashcards
How can percentage errors be reduced?
Use larger quantites (volumes, masses etc)
Use more precise equipment
How can procedural errors be reduced when making a standard solution?
- Rinse out weighing bottle with distilled water and add this to the volumetric flask
- Ensure the bottom of the meniscus is level with the graduated mark
- Invert the flask many times to ensure complete mixing of the solution
How can procedural error be reduced when carrying out a titration?
- Rinse out pipette and burette with the solutions that are going in them.
- Swirl the flask during the titration
- Rinse the side of the flask with distilled water
- Add the solution in the burette dropwise near the end point
What is the setup for collecting a gas?
What are the issues with the methods of collection gases?
Gas syringe:
- Syringe can stick due to the damp gas
Collection over water:
- Some gas dissolves in the water
Both:
- Some of the gas escapes before the bung is inserted
How can insoluble impurities be removed from a sample?
- Dissolve the sample in a suitable solvent
- Filter the mixture (rinse the residue on the filter paper with a little solvent)
How can dry crystals of x be produced from the solution?
- Put the solution in an evaporating basin and heat gently
- Evaporate half and then allow the rest to evaporate at room temperature
- Transfer the crystals on filter paper and wash with ice cold water
- Place the filter paper in an oven to dry out the crystals
What precautions should be taken if chemical x is an irritant/corrosive/toxic?
Wear gloves/goggles
Avoid contact with the skin
What precautions should be taken if x is flammable/volatile?
Avoid naked flames
Why is recrystallisation necessary?
- Chemicals may be impure
- Chemicals may decompose over time to form other compounds
- The reaction may not go to completion
How is a chemical purified by recrystallisation?
- Dissolve in a minimum volume of hot water
- Filter solution hot
- Cool filtrate in ice
- Collect crystals and wash with cold water
When carrying out a recrystallisation, why is a minimum volume of hot water used?
- To obtain a saturated solution
- To increase yield (reduce amount left in solution)
- Enable crystallisation on cooling
When carrying out a recrystallisation, why is the solution filtered hot?
- To remove insoluble impurities
- To prevent crystals forming during filtration
When carrying out a recrystallisation, why is the filtrate cooled in ice?
To increase the amount of crystals that are formed
When carrying out a recrystallisation, why are the crystals washed with cold water?
To remove soluble impurities
How can the purity of a product be tested?
Melting point determination
Why do pH meters have to be calibrated before use?
Over time the meters gives inaccurate readings
Before recording the pH with a meter, how would you ensure the reading was accurate?
Calibrate using a buffer solution of know pH
Adjust the meter accordingly
How does the purity of a sample affect its melting point?
Impure samples with melt of a wide range
Pure samples will melt “sharply”
Why might the melting point of a sample be lower than the data book value?
- Temperature in thermometer lower than in capillary tube
- Sample too small
- Heating too quickly
What is the setup for filtration under reduced pressure?
What is the setup for reflux?
What is the setup for distillation?
Why might a reaction have a low percentage yield?
- Incomplete reaction
- Transfer losses
- Gas escaping
- Product dissolving (either gas or a salt)
