Energy changes
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In chapter 5 (Energy change), when we use “q = mcT” in the first stage of determining enthalpy change, does “m” refer to the mass of original solution or do we add the mass of solute to solution? (e.g. 1.98 g of Mg to 50 cm3 HCl ).
Igloo writes
If we were mixing solutions together, we would undoubtedly use the total mass of mixture, which approximately is numerically equal to the total volume, since the density of aqueous solutions can be taken to be 1.0 g cm-3.
In your example, using 50 g instead of 51.98 g is a reasonable step to take. The metal dissolves to form a salt in solution. So in effect you are making the usual assumption that the denisty and specific heat capacity of the mixture are the same as the values for pure water.
There are several other errors involved, such as the transfer of energy to or from the surroundings in experiments such as these. Therefore there is no point fussing over quoting the mass of solution to 4 significant figures, when 2 significant figures would be far more appropriate. Incidentally this is why final answers to these calculations cannot be quoted to more than two significant figures.
In any Nuffield Chemistry examination paper the question will be phrased in such a way as to make clear how you are meant to quote the value for “m”.
Risk assessment
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For further guidance see our tutorial on Risk Assessment.
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updated: 26 August 2004
