For paper 5 Flashcards
How to prepare 250.0cm^3 of FA1 from the weighted sample of FA2
Dissolved FA2 in distilled water. Transfer the solution to a 250^cm3 volumetric flask and fill to 250cm3 mark with distilled water. Invert and shake the flask to homogenise the solution
Percentage error
(uncertainty / measured ) x 100
How to improve accuracy of the measurement with balance scale
Use a balance scale which records more than 2 d.p
How to improve the reliability of the experiment
Repeat the experiment and check if consistent results are obtained. Calculate the mean
How to improve accuracy of the measurement when experiment uses upside down measuring cylinder to measure volume
Use a gas syringe and with smaller divisions
What is meant by a standard solution
A solution of known concentration
Why is conical flask not shaken during the final step
To allow the solid to settle
Why is a larger titre better than a small titre
Smaller percentage error
How can a student improve the reliability of the experiment
Repeat the experiment and calculate the mean
Why do you need to reweigh and heat until mass is constant
To ensure that all the water has been removed from the solid. (So that the mass obtained is the mass of the precipitate only)
Why do you need to wash the precipitate with distilled water
To remove unreacted (e.g sodium chloride)
Why do you need to filter the solution a second time
To collect any precipitate that passed through the filter paper the first time
Percentage yield
(actual yield/ theoretical yield) x 100
Which qualitative test can be used to distinguish an aldehyde from a ketone
Add Tollen’s reagent. Silver mirror formed on the test tube for an aldehyde. No change for ketone.
Add Fehling’s reagent. Blue solution turns red for an aldehyde. No change for ketone
What general experimental error may produce an anomalous result
- Incomplete reaction
- Control variable not controlled properly
- Human error in measuring quantities and reading measurments
- Evaporation of some of the products/reactants
How could you confirm that a result is anomalous
Repeat the experiment for that value twice more. If the two new values match, it can be concluded that the reading was an anomaly
Suggest why the precipitate is dried in an oven and not by direct heating with a bunsen burner
It is dried in the oven to avoid thermal decomposition of precipitate
Other than by changing the balance, state how the percentage error could be reduced
Use a larger mass of solid
Suggest how a student could ensure a solid dissolves as quickly as possible
- Stir with a glass rod when solid is mixed with water
- Increase the temperature of water
What precaution should a student take when dealing with chemicals that is corrosive to the skin
Wear chemically resistant glove
Suggest how the student could ensure that they collect pure oxygen gas in the conical flask
Do not collect the first gas to be collected
