A Chemical Balance W. M. SPICER and J. D. RAULERSON Georgia School of Technology, Atlanta, Georgio
the most important instrument used by PERHAPS chemists . is. the balance. From a historical stand-
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point this is certainly true. In spite of this, chemists have not developed a true chemical balance, i. e., one that weighs directly in terms of chemical units of quantity, moles or equivalent weights. To a physicist, 1g. of sodium hydroxide is equivalent to 1 e. of ~otassinmhvdroxide. But to a chemist. 1 e. of sodium hydroxide is not equivalent to 1 g. of potassium hydroxide: to him 1mole of sodium hydroxide is equivalent to 1mole of potassium hydroxide. It is almost certainly true that one cannot think quantitatively in chemistry until one thinks not in terms of grams but in terms of chemical units, moles and equivalent weights. We try to teach our students to think in terms of such units. For example, we tell them that a molar solution is made by dissolving 1 mole of solute in enough water to make 1 liter of solution. This is a simple straightforward definition but it is not directly applicable in practice. When the student goes into the laboratory to prepare solutions he is told that in order to make 1 1. of 1 M solution he must use 40.0 g. of sodium hydroxide, 74.0 g. of calcium hydroxide, 58.45 g. of sodium chloride, or 169.89 g. of silver nitrate, etc. This is very confusing to
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Furthermore, students quickly learn to interpret chemical equations in terms of moles. They learn that the equation NaOH
+ HCI = NaCl + HsO
expresses the fact that 1 mole of sodium hydroxide reacts with 1mole of hydrogen chloride to yield 1 mole of sodium chloride and 1 mole of water. But if they are asked the size of a mole of sodium chloride, they usually, after some hesitation, give an unreasonable answer. The student is not t o be blamed; he has quite likely never been shown a mole of sodium chloride, or a mole of anything else. We talk a lot about moles but we seldom deal directly with them. In an attempt to bridge the gap between class discussion where we use moles, and laboratory practice
where we are forced to weigh in grams, we have constructed a simple balance and accompanying weight that weighs directly in moles. The balance (see figure) consists of a pan, A, to receive the sample, an accurately calibrated arm, B, to receive the weights and to cany the rider, C. This rider is used to balance the container in which the samale is weiehed.
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Each substance to be weighed requires its own particular weight proportional to its molecular weight. (Our weights are two-thirds of the gram-molecular weight.) In the figure three such weights are shown. Each of these when placed a t any given position on the balance a n n (say 1.5) will balance that number (1.5) moles of the given substance on the balance. Thus to weigh 1.5 moles of any substance, no matter what, we use the 1.5 position on the balance ann. It is not, of course, expected that this type of balance will replace our present analytical balance. For one thing, its sensitivity is low, but a more serious disadvantage is that too many weights are required. In spite of these shortcomings, we believe that such a balance has an educational value and might be useful too where a few substances are weighed often.
