In ancient times the perfect gas equation was used to determine molar masses \(M\) [g/mol]
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| \begin{equation*} p\,V = n\, R\, T = \frac{m}{M} \, R\, T \quad ; \end{equation*} | (1.8) |
e.g. fixing \(V\) (known flask), \(T\) and \(p\) (1 atm), the determination of \(m\) with the aid of a balance (very precise) allows to calculate \(M\) using the above equation. Thus the determination of a molecular property is possible without knowledge about the molecules. For example such measurements allow to characterize aggregates, e.g. S\(_8\). The density and vapor pressure of sulfur at 500 \(^{\circ}\)C was determined to be \(\rho =\) 3.71 g/l and 699 Torr, respectively. To identify the kind of molecules present in the vapor we take into account that \(M = \rho\,R\,T / p = 256\) g/mol must be a multiple of the atomic mass of sulfur (i.e. 32 g/mol), thus obviously S\(_8\) is the species in the vapor phase.
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© J. Carstensen (TD Kin I)