mass
'no magic to the mole'
amount
molar mass
concentration
solution volume
gas volume
molar gas volume
Avogadro
constant, L
number of
entities, N
Now try the following question.
X4. THE S.I. FUNCTIONS WITH QUANTITY CALCULUS
In any event, however, derived physical quantities such as molar mass, volume, concentration, and molar gas volume come into play quite swiftly.
Direct measurements can be used in procedures to obtain amount of substance indirectly. The table below summarises the relationships that might be required. The SI units that are met most commonly are highlighted; these need to be manipulated competently when physical quantities are substituted in place of quantity symbols.
Unlike mass, length and time, however, physical quantities involving amount of substance and its SI unit symbol mol have - initially at least - far less physical significance for almost all of us than units like kilograms (kg), metres (m) and seconds (s). These we all experience first hand on a daily basis, whether we like them or not, and assume that we know what they represent.
Due to the abstract nature of physical quantities like amount of substance, molar mass, and molar gas volume, not to mention the various mass and volume unit conversions necessary as a matter of routine, attempting to execute calculations without including SI unit symbols is a recipe for disaster.
As should be forthcoming from the table above, with many new units being met in quick succession, how do students make sense of what is occurring while looking at a page of numerical values, where unit symbols are in short supply ? This is one of the principal reasons why these calculations present far more difficulty than ought to be the case. The SI has been developed for very good reasons: it should not be ignored; for science educators to do so is nothing short of an abrogation of responsibility.
AMOUNT of SUBSTANCE.com offers the necessary grounding in using correctly the vitally important relationships attached at the site header, like n = m / M & c = n / V. Solving problems involving these physical quantities should not be a case of randomly multiplying or dividing a couple of numbers together and perhaps hoping for the best - that is the level expected of a poorly instructed seven-year old in an arithmetic class.
Far too often, however, this happens to be the case in the lab when pupils are being introduced to calculations involving amount of substance. The fact that they might well not be being schooled in the correct use of SI units during a physics class while tackling a density calculation, or a maths class while processing a volume calculation, does not make the task for the chemistry teacher any easier.
In particular, the reluctance of the vast majority of teachers, instructors, text-book authors and
examiners in embracing the SI, is the principal reason why attempts to deliver a coherent curriculum framework that addresses the physical quantity amount of substance, coupled skilfully with stoichiometry, are more than likely to flounder. Routinely, for a given physical quantity, most students remain unconvinced of the purpose of associating a numerical value with the appropriate unit like g / mol, mol / L, or L / mol.
Has the time not arrived where pupils dealing with calculations involving physical quantities - whether in mathematics, physics or chemistry - are presented with the coherent SI approach ?
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