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Why can’t my students work out percentages in chemistry?

This is a common difficulty as students find it hard to transfer skills from one subject to another. It is always helpful to remind students what a percentage is, before asking them to carry out calculations in a chemical context.

Can you recommend a good experiment for teaching percentage yield in the classroom?

There are a number of different experiments that could be used but making hydrated copper(II) sulfate is relatively straight forward for younger students. Synthesis of aspirin or the nitration of methyl benzoate or biodiesel would be suitable for more advanced students.

What about the uncertainty in experimental results? Should these be quoted in percentages?

The uncertainty in experimental results is often quoted as a percentage as this is more meaningful. From this value, the limits of experimental uncertainty can be easily worked out without having to know what equipment was actually used during the practical work. It is good practice to encourage students from an early age to start thinking about sources of error and the impact they could have on the final answer. This in turn wil help them to develop their evaluation skills.

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