Big Idea #5: Exam Technique

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Exam questions on chemical analyses (especially titrations) are designed to make you think and test whether you can pull out the key bits of information relevant to a calculation. There will always be lots of detail that you can safely ignore and even very long questions could probably be condensed down to a few bullet points.

The key bits of information in a calculation always relate to (or can be related to) the moles of substance. Here are the key ones to watch out for in any question. 

  1. A mass of substance
  2. The concentration and volume of a solution
  3. Any information on sampling and dilution
  4. The pressure, volume and temperature of a gas
  5. The balanced symbol equation

Here is an example:

Q1: A commercial type of cleaning product is sold in 500 cm3 bottles and contains H2O2 as the active ingredient. A 25 cm3 sample of the cleaning product was transferred by pipette to a 100 cm3 conical flask. The amount of H2O2 was the determined by titration with 1.0 mol dm-3 acidified MnO4– solution from a burette. The solution changes from colourless to pale pink at the end-point due to the presence of unreacted MnO4–.

5 H2O2 + 2 MnO4– + 6 H+ –> 2 Mn2+ + 8 H2O + 5 O2

The titre volume was found to be 19.7 cm3. Determine the % mass of H2O2 in the cleaning product. Give your answer to 2 significant figures.

There is a lot of filler in this question so we need to read through all the detail and pull out the key bits of information. Only the details in colour are used directly in the calculation.

  • n MnO4– = 1.0 x (19.7/1000) = 0.0197 mol
  • H2O2 reacts with MnO4– in a 5:2 ratio
  • n H2O2 = 0.0197 x 5/2 = 0.0493 mol in sample
  • The total is 500 / 25 = 20x bigger than sample
  • n H2O2 = 0.0493 x 20 = 0.985 mol in total
  • m H2O2 = 0.985 x 34 = 33.5 g in total
  • m total = 500 g
  • % H2O2 = 33.5 / 500 x 100 = 6.7%

If you can identify and pull out the key bits of information, you will start to find that calculation questions become much more repetitive and straightforward. You will notice that you are doing the same few steps every time.

The general sequence to many questions will be:

Step 1:
Use the data in the question to find the moles of one substance.

If starting from a known mass of a pure substance, use n = m / Mr
If something to do with solutions, use n = cv
If something to do with gases, use n = pV /RT 

Step 2:
Use the balanced symbol equation, and information on sampling and dilution, to find the moles of another substance.

Step 3:
Finally, do something with your answer (e.g., find concentration mass, Mr, purity, x, etc!)