A level paper 3 mistakes Flashcards
Define relative isotopic mass.
The mass of an atom of an isotope.
Relative to 1/12th of the mass of a carbon-12 atom.
How would you draw a diagram of the apparatus used for filtration under reduced pressure.
Conical flask with side arm labelled ‘to pump’
Buckner funnel with perforations.
Bung around the neck of the flask.
Flat filter paper over the perforations.
describe the recrystallisation process to obtain a pure, dry sample of paracetamol.
Dissolve crystals in the minimum volume of hot solvent.
Filter hot and allow to cool.
Filter and wash with a small amount of cold solvent.
Dry crystals in a warm oven.
Describe what happens to the melting temperature of paracetamol if it is not pure.
melting temperature is lower.
Melts over a range of temperatures.
In an experiment where you add sodium carbonate to HCl and measure changes in temperature.
Describe changes to the method to improve accuracy using a clock and graph plotting.
Measure temperature every 30 secs for 2.5 mins.
Add sodium carbonate
Stir and measure the temperature every 30 secs for another 5 mins.
Plot a graph of temperature against time.
Extrapolate the lines for the 2 sets of points and determine the maximum temperature change.
Explain why the pH of a 1x10^-8 solution of nitric acid is not 8.
Hydrogen ion concentration from water is 1x10^-7.
So total H+ is greater than 1x10^-7.
In the practical where a hydrated salt is heated in a crucible to remove water of crystallisation, what can be done to tell when all of the water of crystallisation has been lost.
Heating to a constant mass.
Why are anti-bumping granules added to the reaction mixture?
To promote smooth/controlled boiling.
Is the reaction between sulphuric acid and an alcohol exo or endothermic?
exothermic.
Why might anhydrous sodium sulphate be added to a reaction mixture?
To absorb water
In a recrystallisation practical, student 1 had a very high yield and student 2 had a very low yield.
Explain possible reasons for these results.
Student 1 - Crystals were’t dry when weighed.
Impurities in the crystals.
Student 2 - Unwanted side reactions.
Not all crystals had formed some left in solution.
Some crystals left on walls of container when transferred.
Define the term relative atomic mass.
The weighted mean of the masses of its atoms relative to 1/12 th of the mass of a carbon-12 atom.
When determining the number of moles of water of crystallisation, a student forgot to use the lid on the crucible.
State how this mistake would affect the calculated value of n.
More salt lost as it escapes crucible.
So n greater as water of crystallisation greater than expected.
When determining the number of moles of water of crystallisation, a student heated for 1 minute instead of 5.
State how this mistake would affect the calculated value of n.
Less water of crystallisation removed through evaporation.
So n less than expected.
What reagent is required for dehydration?
Concentrated phosphoric acid.
How would you draw a labelled diagram of the apparatus for distillation?
heat source, flask with reaction mixture and anti- bumping granules, still head and thermometer opposite ‘exit’
(still head is the thing that goes into pear shaped flask)
Downward angled condenser attached to the side arm. Water in below water out.
Collection vessel.
When recrystallising, why may the filter paper and funnel be warmed in an oven before use?
To make sure the solution doesn’t cool significantly.
To prevent crystallisation taking place on filter paper.
Why may the filtrate be left to cool and recrystallise during recrystallisation?
Product less soluble in cooler solvent so will crystallise.
What does filtering under reduced pressure do?
removes soluble impurities from the filtrate.
devise an experiment to determine the Ka of ethnanoic acid with an unknown concentration.
You have a pH meter and sodium hydroxide.
Titrate ethanoic acid with sodium hydroxide.
Measure pH at regular intervals.
Plot a graph of pH against sodium hydroxide added.
Use graph to find half equivalence point.
At half equivalence point pH=pKa so Ka=10^-pH
