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Calculations are an important part of many pieces of practical work, but some students find the Maths required difficult. I shall try to persuade you that the Maths you need really is quite simple. The key to all these calculations is to understand the principles behind them. The same ideas are used in many everyday calculations which few people have difficulty with. I shall also include a number of worked examples which you can practise with. |
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There are some general principles that apply to all calculations. Even a correctly carried out calculation can lose a lot of credit if you forget, or make mistakes with, the units, sign and significant figures. Always think U.S.S. at the end of your calculation (Units, Sign, Significant figures).
With the more difficult multi-stage calculation, it is always much easier if you have seen it before and can remember all the stages. However, you will meet new calculations (or at least variations of the problems you have seen before). This often panics candidates, and they sit waiting for some inspiration which will solve the whole problem at once. This inspiration rarely arrives and the calculation remains unattempted. This is a disaster in an exam or an assessment as it will score zero. Don't be afraid to make a start on a calculation, even if you are not sure where the calculation is heading. Often, as you progress, the final solution becomes apparent. Have a look at the following example:
0.368 g of compound X of molecular formula C7H6O2 was treated with excess at this point the calculation becomes very confusing and you really don't know what to do. If you've taken the trouble to translate this section, then well done!
Most of this makes no sense, but is there anything you can pick out which would enable you to demonstrate that you have some calculating skills? One thing that we can see is a molecular formula, so we could calculate a molar mass. When you do this make sure that you label your calculation carefully. Having 122 scribbled in the margin is unlikely to gain any credit, but writing down molar mass of X = 122 g mol-1 may well get you a mark. Similarly, a labelled calculation for the number of moles will probably get another mark, number of moles of X = 0.368/122 = 0.00302 mol. The other benefit of careful labeling is that it makes it much easier for you to bring all the pieces together and come up with the final solution.
Another good starting point is to find a concentration and a volume of a substance as this also enables you to work out the number of moles using the following equation:
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Number of moles (in mol) = concentration (in mol dm-3) × volume (in dm3) |
The following links show the typical calculations that you will meet at "A" level:
