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The ability to lay out calculations in a comprehensible manner depends on

        Teachers I have spoken to often like my list less the further they go down it. The last item in particular raises comments such as 'my students have enough problems without bringing in this idea.' My response is that the problems can arise because they don't use this idea! Like most worthwhile things it involves an initial effort that makes succeeding work much easier, so overall needs less work to achieve a given end.

        The advantages of using units throughout include:

Here is a simple titration example to show what I mean, easily extended to other types of calculation.

 

25.0 cm3 of a solution of sodium carbonate was titrated with 0.108 mol dm-3 hydrochloric acid solution using methyl orange indicator. The volume needed was 27.2 cm3.

Find the concentration of the sodium carbonate solution in mol dm-3, and in g dm-3 of the anhydrous salt.

 

        The reaction is: Na2CO3 + 2HCl 2NaCl + H2O + CO2

        Amount of hydrochloric acid used = 0.0272 dm3 0.108 mol dm-3

                                                            = 0.00294 mol

        Thus amount of sodium carbonate = x 0.00294 mol = 0.00147 mol in 25.0 cm3

    Thus concentration of sodium carbonate solution = 0.00147 mol /0.025 dm3  

                                                            = 0.0588 mol dm-3.

    The molar mass of anhydrous sodium carbonate is

                                {(223) + 12 + (316)}g mol-1 = 106 g mol-1;

    The concentration of the sodium carbonate is therefore

                                0.0588 mol dm-3 106 g mol-1 = 6.233 g dm-3.

 

Note the following, which I promise you will increase your understanding and your marks if you do them too:

You might think that this is quite a performance for a relatively easy calculation. Part of your apprenticeship is, however, learning to solve problems using in-built checks whilst also realising what you're doing. This method achieves both things.


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