Nomenclature and Symbolism for the "Quantities" of a Substance Peter Glavic University of Maribor, YU-62000 Maribor. Smetanova 17, Yugoslavia The mole was adopted in October 1971 as a base unit in the International System of Units ( I ) . This fact has enabled us to develop a better system of quantities and units, which is gaining increasing international acceptance (2). I t was soon acknowledged that this system was simpler than the variants that had existed before. But it is evidently not sufficiently known that besides the names and svmbols for units the name and symbols of physical quantities are also the subject of systematic study; this should bring some further advances and simplifications. We use about 2000 physical quantities today, and therefore systematics and proper nomenclature are necessary in order to give young people a simple and efficient way to memorize the quantities and units. l ' h e interdisriplinary and multidisciplinary character of modern science and technolow reouires a loeical and unified a ~ n r o a c hto the svmbolsTor physical ¶&tities (3-5). chemists, who had iong aeo recoenized the im~ortanceof svstematics (in the ~ e r i o d i E s y s t e i of elements) and nomenclature (in both brganic and inorganic compounds), should take part in this development. Therefore, it seems appropriate to look at what has been achieved until now: what are the problems in the use of quantities and units and where we should go in the future. Ouantlty and Amount of Substance A given quantity of substance can be expressed in several ways (6): as a mass, m as a volume, V
asan amount of substance', n as a number, N
m(Hz0) = 1 kg V(HzO)= 1dm3= 1L n(H10) = 55.6 mol N(HzOmolecules) = 33.5.1OX
I t is apparent that expressing the quantity by the number of its molecules is cumbersome. Apart from the needs of stoichiometrv. this was the main reason for introducine the mole as a base ;nit. The mole changed our molecular quantities to the macrosco~iclevel. So there is no need touse Boltzmann's constant-we have substituted it with the gas constant: k.NA=R Table 1 presents some other transformations of this kind. Macroscopic values make more sense since we are never dealing with individual entities and most of the properties are statistical values of large groups of entities. Table 1 indicates that the mole as a base unit enables several constants to be expressed by normal SI prefixes. The same arguments are valid for some quantities derived from amount. "Molar mass" may be substituted for old quantities such as "molecular mass", "atomic mass", "formula weight", etc. Therefore, we should write, for example: M (H) = 1.00794 glmol
Constam slecmn mass
newon mass Plancvs constant Bohr magneton Dipole moment
Molecular quantlty
Molar quantity
9.109.10-3' kg 1.675.10-17 ko 6.626.10-" J:. 9.274.10-z4J/T 3.336.t0-'0 Gm
548.6 pg/mol 1.009 dm01 399.0 pJs/mol 5.585 Jl(T.mo1) 2.009 Cm/kmol
Also, "relative molar mass" is more appropriate than "relative molecular mass". In the chemistry and technology of gases we have frequently used the quantity "normal volume" with the unit "normal cubic meter", mn3.The volume of a gas depends on external circumstances and is not appropriate for chemical reaction kinetics and chemical engiueeriug calculations. Therefore, we recommend using the amount (of gas) instead; the relationship is simple: m.3 = 44.6 mol for an ideal gas (8). Ouotlents of Physlcal &antities If we divide one quantity by another quantity a quotient quantity is obtained. For example, let us take mass and volume. If we divide mass by volume we obtain a quotient that is called "density". Conversely, if we divide volume by mass, we obtain a quotient that is called "specific volume". In the first case, the corresponding SI unit is kgIm3 and in the second case it is m3kg. International and national organizations for standardization are trying to systematize the nomenclature of these quotients. Table 2 gives some examales that are common in chemistrv. By dividing two identical quantities, we obtain ratios (9). The ratios with the exce~tionof number ratio 12) are shown on the diagonal of ~ a b l ~ 2 .
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Composltlon 01 Mlxed Phases Chemical samples often contain two or more substances. Their . oro~erties denend on comnosition: therefore, we verv . often express their composition using four different quotients: fractions. ratios. concentrations. and molalitv. We shall limit our discussion to homogeneo& gaseous, liquid or solid mixtures (101, but, depending on the context, we can also use the same terms with heterogeneous or multiphase systems. We can also express the elemental or componental composition of samples in the same way. The composition of mixed phases can be expressed by the mass of one component, rnB, its volume, VB, its amount, ng, or its number NB,in comparison with another component (ma, VA,na, NA,respectively; with total mass, m = Elhm,; with total volume, V = ElkV; with total amount, n = Elkn;; and with tatal numher, N = E1kNj,k being the number of components.
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The words "of substance" may be replaced by the specification of the entity, e.g., amount of H20 (7). 130
Table 1. Some Consequences of the Adoptlon of Mole In Several Constants
Journal of Chemical Education
If the total volume before mixing is Vo and the total volume after mixing is V, then in general Vo + V. The quantity VB
Table 2.
Simple Quotients of Some Quantltles and Thelr Symbols (Not Appllcable to Mlxture) Numerator
Denominator
Mass, m
Mass, rn
Mass
r
ratio, Volume. V Amount. n Time, f
Volume, V
Amounta, n
Specific
specific [amount]
specific work, w
specific heat. q
specifi~ enthaipy, h
Molar enthalpy. H,
volume, v
Heat, 0,
Work, W
(Mass)
Volume
density, p
ratio, $
Molar mass, M
Molar volume, V ,
Amount ratio: r
Molar work. W,
Molar heat, 4
Mass
Volume flow
Amount 'flow
POWBLP
flow
Heat tiow
rate. q,
rate. q,
rate. F
Enthalpy. H
rate. ?
rate.
'Amount Isan abbreviationfathe"amountofsubstan~e": slnoewe hsvetodefinetheentltle~ln~~Ived, we csndo itsimply wimthe entltysymbol l e g , n 8)orihnameie.g.,amoumof 3. amount density of 6).
Ususiiy erronsously named "mole ratio". oOusntiliesin brackeh are n n yet adopted by IUPAC a IS0
can be used only in the case when the entity B does not change its state. We have alreadvdiscussed the ratios (Table 2). Therefore. let us take just one example of theobso
