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.
X1. RATIONALE FOR AMOUNT of SUBSTANCE.com & STOICHIOMETRY.co.uk
The purpose of AMOUNTofSUBSTANCE.com is to provide, in a logical order, the material necessary to develop or/& improve fluency when working with the physical quantity called
amount of substance, whose SI unit is the mole, and which adopts the SI unit symbol mol.
There is certainly no magic to the mole, the SI base unit for amount of substance, in the same way that there is no magic to the second, the SI base unit for time, or the metre, the SI base unit for length. In carrying out calculations in maths - of the type, say, average speed = distance / time, or in physics - of the style, say, mass = density x volume - we do not concern ourselves with formal definitions of the base units metre, second and kilogram. Undisturbed by such definitions, almost all of us could proceed to tackle the couple of problems set out below as worked examples.
For the gentle calculations that neophytes should meet on being introduced to the new physical quantity amount of substance and its corresponding SI unit mole - SI unit symbol mol - what is being attempted, and the techniques involved, should be no more difficult than those presented in the pair of calculations above, providing one can also add up correctly in order to take care of molar masses.
Use of the relationships below, as appropriate,
enable us to inter-convert mass, m, and amount of substance, n,
by making use of the bridging quantity known as molar mass, M.
The reason why such conversions need to be carried out frequently in chemistry is that, in practice, mass, m, can be measured directly - on a top-pan balance in the lab, for example - but, in any routine fashion, amount of substance, n, cannot. For a substance whose identity is known, obtaining its molar mass, M, from data tables should be straightforward.
Efficiency in these conversions, e.g., mass ® amount, is based on correct techniques, which involve use of what are known as quantity symbols, namely the italicized, single letters like M, n & m, used as a matter of convenience to represent the various physical quantities, and that we can perform algebra upon, as is often necessary.
It will be seen that successful solution of introductory problems does NOT depend upon knowing the definition for the SI unit mole. Pupils may already have had the misfortunate of being bamboozled when told that the mole is a mass (incorrect), or that the mole is an incredibly large number (incorrect). Looking puzzled often brings forth from an instructor a multiplicity of unhelpful analogies, most of which are frankly ludicrous; and for good measure, an Italian physicist is often dragged into the mix also, in the shape of poor old Avogadro.
Let us be absolutely clear what the mole is NOT, however:
1. it is not a physical quantity; so while one might refer to mass, volume, and concentration, one cannot legitimately - in the same breath - talk about moles. But a majority of educators continue to do just that, taking ‘moles’ as a pseudonym for the physical quantity amount of substance.
As a consequence they introduce an unfathomable layer of complexity for their charges.
2. it is not a mass. How could it be ? Not being a physical quantity, it can't. Otherwise it should be measured in g or kg, etc.
3. it is not a number. Otherwise it could not be a base unit in the international system of units (SI).
In practical terms, the mole is the SI unit for the physical quantity called amount of substance.
At this point, students are advised to avoid the next three pages - which deal largely with teaching strategies - and instead jump to an overview of the content of this pair of sites and best navigation of Amount of Substance.com and Stoichiometry.co.uk. Alternatively, head here to the beginning of the section dealing with the meaning of chemical amount. You can always return to this page by clicking on the iconical flask attached to the site header.
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