Measuring it out, one mole at a time

NS Sundaresan

Most chemistry teachers are familiar with the mole concept which is introduced to students only in the tenth class. This article makes a case for rectifying this anomaly and reviews the usual way of teaching this concept.

The mole, as every chemistry teacher knows, is a unique chemical quantity, something that is the basis for every chemical reaction. Using the mole as a unit we are able to show the rationale behind chemical reactions, deduce stoichiometry, write the so-called “chemical equation”, perform required reactions quantitatively without wasting chemicals in trial and error methods. Such an important concept has to be obviously taught quite thoroughly early in the formal study of the subject. Unfortunately, in our school curriculum we first introduce chemical equations with a certain arbitrariness after introducing elements, compounds, etc. Only much later do we introduce the mole in the form of a disjointed chapter of numerical calculations. We introduce the laws of chemical combination even later (11th grade). Therefore the chemical equation does not appear as a natural consequence of the laws of chemistry. While we teachers cannot change syllabi, we can perhaps rectify this anomaly by introducing the mole concept even during classroom demonstrations at the eighth grade level.

The classroom demonstration can then be turned into a quantitative experiment with the numerical calculations following right after it. Of course, I do not deny that we should introduce the sights and sound (the colours and the smells) first. But this can be done at earlier levels without burdening younger students with numbers.

In this article, I suggest a few well known experiments as experimental support for the mole concept. Of course that large pool of resources – the internet – has many experiments to offer. (Some links that you can access on the internet have been given at the end of this article). Here I would like to suggest only the well- known text book experiments which can be easily carried out in the average school classroom. These experiments are already well-known to chemistry teachers. My point here is to suggest the appropriateness of doing the experiments quantitatively in the 8th grade, so that the mole concept can be quietly introduced alongside. The extensive numerical can follow naturally in the 10th grade as usual.

At this stage it is also relevant to review the usual way of teaching the concept and avoid some of the pitfalls found in our text books. Here is a suggested sequence for the introduction of the various points. Chronology is important but not exclusively so. The experienced teacher knows what questions are coming from the flock!

  1. Survey of experiments of the 18th and 19th century chemists – the idea of elements and compounds. The experiments of Lavoisier, the combination gases in simple proportions (Gay Lussac-Charles law) as also Avogadro’s hypothesis. These lay the foundation nicely.
  2. The students are quite likely to ask as soon as the mole is mentioned, “How was the quantity arrived at and how was the Avogadro number found?” Here we are on tricky ground. Hence it is better to mention Dalton’s theory which brings in the small size of the atom. Thus the idea that even a small mass can contain a very large number of atoms can be brought in. Berzelius’ determination of relative atomic masses can also be mentioned. Later if necessary, introduce the Loschmidt number and then the Avogadro number.
  3. Now it can be shown that it makes sense to rationalize combining masses of substances in terms of new quantity viz; the mole, as one can express them as simple whole numbers.
  4. The above naturally leads to the idea of the chemical equation.

Discussing the points above mentioned will probably take two classes, after which we are ready for the experimental demonstration. Given below are the experiments.

Experimental set-up for heating

Experimental demonstrations

Heating of copper (II) carbonate:
This is quite well known and comes in the ninth grade practical syllabus under “Action of heat on substances”. This can be nicely turned into a quantitative experiment. A set up shown here will serve the purpose.

Since we have to weigh certain quantities, the following precautions have to be kept in mind*

  • The amount of substance has to be carefully chosen since the mass change has to be significantly large to be seen properly in the balance (which should preferably be a digital top loading one). However, too large an amount will take up too much time to heat and also the carbon dioxide produced should not exceed the capacity of the absorbent. If this is lime water then the formation of soluble calcium bicarbonate must be avoided. Hence it is recommended that 0.01 mole or 1.23 g of copper carbonate be taken and the substances weighed separately in butter paper and then transferred to the heating tube. Similarly the CuO formed should be removed and weighed in a butter paper. The weighing of the lime water tube may present difficulties and is therefore optional.
  • Perform the experiment once or twice yourself before taking it to the classroom.

After the experiment the chemical equation can be written and calculation done in terms of moles.

A second experiment can then be performed involving the heating of copper (II) nitrate. The experiment is very similar, with the difference that NO2 produced need not be absorbed but rather shown as a brown gas. Here also 0.01 mole of Cu (II) nitrate can be taken.

After this, it can be very easily shown how, in terms of mass alone it is difficult to rationalize the fact that different masses of substances produce the same mass of CuO. By contrast it makes more sense if we look at the chemical equation in terms of moles. Later, it can be mentioned that since mass is the measured quantity, we derive the mass – mole relationship. It is also a good idea to actually show the students the masses of one mole of various substances (elements and compounds).

Since this article addresses chemistry teachers, a full detailed procedure has not been given. It must be emphasized that care must be taken at all stages of the experiment. In addition the limits of experimental errors must be mentioned.

Readers interested in trying out more experiments can access the following site on the internet:
www.freewebs.com/gozobiochem/chapter%206

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*The mass of substance being heated is small; therefore the final mass will be even smaller. Since the mass of the test tube will be much larger, it will be difficult to see the change in mass if the substance is weighed with the test tube. Hence I have suggested use of butter paper. Of course there will be some difficulty in transferring from the test tube and there may even be some residue sticking to the test tube. But this has to be explained as the limitation of the apparatus. Also, the limits of experimental error must be clearly explained.

The author is a retired teacher of chemistry. He has taught chemistry at various levels from the high school to the post graduate level. He can be reached at kaone52@gmail.com.