One of the best aspects of Chemistry A level is that you get to do lots of practical work 😎.
Analysing unknown compounds, carrying out titrations, designing kinetics experiments, synthesising and purifying organic substances all help to develop your manipulative, observation and critical thinking skills.
But … you know there was a BUT coming … questions on even the most simple of practical techniques are always very poorly answered!
Here’s what the examiner’s reports from OCR and AQA last summer had to say on the matter –
Let’s take a look at a couple of common exam questions and figure out the best way to tackle them …
Case study: titration
Question: A student plans to find the concentration of a solution of nitric acid by titrating it against a standard solution of potassium carbonate.
While making up the potassium carbonate solution, the student spilt some of the potassium carbonate on the bench as they tried to transfer it to a beaker from a weighing boat. Not all of the solid that had been weighed out was dissolved to make up the solution.
How will the concentration of the nitric acid calculated from the results of the titration using this solution compare with the actual concentration of the acid. Explain your answer. [3]
Exam technique tip:
The trick here is to imagine you are carrying out the practical and work through each of the steps in the titration.
• the contents of the beaker would be transferred to a volumetric flask and made up to the mark – this solution would be pipetted into conical flask with some indicator
• the burette is filled with nitric acid and run into the conical flask until there is a colour change
The potassium carbonate standard solution has a lower concentration than calculated as so it will take a smaller volume of nitric acid (a lower titre) to neutralise it.
This means that the calculated concentration of the nitric acid will be higher than the actual concentration (because we have divided the number of moles by a smaller volume in the final stage of the calculation).
Case study: preparing a hydrated salt
Question: A student is asked to prepare a sample of hydrated magnesium chloride crystals (including the water of crystallisation) .
They add an excess of magnesium oxide to hot aqueous hydrochloric acid and then evaporate the mixture until they are left with a white solid.
Explain why this method would NOT produce hydrated magnesium chloride crystals and give the correct method. [4]
Exam technique tip:
Firstly we need to be clear that this question is not asking us to describe recrystallisation (apparently this was a common error), we simply need to correct the student’s method.
Once again, thinking through how you would carry out the practical is the key.
• we need to filter the excess magnesium oxide from the magnesium chloride solution because otherwise the magnesium chloride crystals won’t be pure
• we should only partially evaporate the water using a hot plate
• and finally we can then leave the remaining water to evaporate slowly at room temperature so we don’t drive off the water of crystallisation
Action points:
Set aside some time this summer to revisit each of the required practicals you’ve done this year.
For each practical you need to:
1. make sure you have a method you can describe in the exam – the best way to do this is to summarise a practical method sheet into 5 bullet points
2. make sure your method includes fine detail e.g. correctly named equipment, the precision of volumes to be measured etc.
3. practise drawing out apparatus / labelled diagrams
4. you need to be able to explain how to analyse the results and complete any calculations
5. you need to be aware of all the common errors in the practical and the effect they have on the final result
6. cross reference your ‘perfect answer’ with a past paper question and mark scheme to double check you have hit all the marking points
If this has been useful then please forward it on and thank you very much for reading,
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