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If I have a substance and want to obtain an exact quantity of it, is it better to measure it as a solid or as a liquid?

In other words, if I have, for example, NaCl crystals I can weigh the crystals in an analytical balance, or alternatively I can dissolve them in water and then determine the concentration by, for example, titrating with silver nitrate.

Which method would be more exact?

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    $\begingroup$ I think your question boils down to whether measuring a mass or a volume is more precise. For the scenario of making a solution, you would have to measure in which volume you dissolve the NaCl, and the titration would involve measuring volumes. On the other hand, if your sample is wet or impure in some other way, the mass would be misleading. $\endgroup$
    – Karsten
    Commented Mar 2, 2019 at 4:03
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    $\begingroup$ You can definitely weigh "better" than you can measure volume. 4 significant figures is the best you can by volume. 4 significant figures is typically a very "good" measurement. Volume measurements are good where multiple samples with the same amount of the substance are required. So you'd make a large amount of a standard solution then take precision aliquots. However it is really hard to weigh out multiple samples with the same 4 significant figure mass. $\endgroup$
    – MaxW
    Commented Mar 2, 2019 at 8:07
  • $\begingroup$ For the specific question asked by the OP, weighing the NaCl using an analytical balance is much more accurate than dissolving NaCl in water and titrating with silver nitrate solution. This is partly due to the latter involving multiple measurement error sources and partly because of the error magnitudes. Of course, this cannot be a generally applicable answer: see the comments by Karsten and MaxW. Also see some of the worked examples in this: EURACHEM/CITAC Guide, “Quantifying Uncertainty in Analytical Measurement”, 3rd Ed., 2012. Also see comment by @theorist. $\endgroup$
    – Ed V
    Commented Aug 17, 2019 at 1:29

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Weighing is always far more accurate than any volume measurement. The extreme example is that of atomic weight (mass) measurements, which required accuracy to 4-5 decimal places. All the work was always done on the basis of gravimetry or by weighing the substances on high quality balances.

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It depends entirely on the substance which you are trying to measure. For NaCl it would be better to dissolve it and then determine the concentration as NaCl is deliquescent it will absorb moisture from the atmosphere thus if you try to weigh it the reading would also include the mass of water absorbed by it. While in some cases it would be more convenient to weigh them to determine their quantity.

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    $\begingroup$ NaCl is usually considered hygroscopic rather than deliquescent, since it's only under unusually high temperature and humidity conditions that it can absorb enough water from the air to dissolve itself. Having said that, the fact that it is hygroscopic, and can thus absorb water from the air, remains. But I'm not convinced that would make a titration more accurate, since there are many sources of error associated with the latter. If water absorption by salt is reversible, it seems it would be better to heat the salt sample in a dry atmosphere to desiccate it, then weight it. $\endgroup$
    – theorist
    Commented Mar 19, 2019 at 17:20
  • $\begingroup$ NaCl reacts with NO2 in the atmosphere . https ://www.nature.com/articles/306676a0 Thus even in dry atmosphere there would be error $\endgroup$ Commented Mar 20, 2019 at 13:33

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