Quantity characterizing the @D01650@ of the @S05746@ from ideal behaviour referenced to @H02783@. The osmotic @C01124@ on a molality basis is defined by: \[\phi =\frac{\mu _{\text{A}}^{*}- \mu _{\text{A}}}{R\ T\ M_{\text{A}}\ \sum _{\begin{array}{c} i \end{array}}m_{i}}\] and on an amount fraction basis by: \[\phi =\frac{\mu _{\text{A}}^{*}- \mu _{\text{A}}}{R\ T\ \ln x_{\text{A}}}\] where \(\mu _{\text{A}}^{*}\) and \(\mu _{\text{A}}\) are the chemical potentials of the solvent as a pure substance and in solution, respectively, \(M_{\text{A}}\) is its molar mass, \(x_{\text{A}}\) its amount fraction, \(R\) the @G02579@ and \(T\) the temperaure. The latter osmotic @C01124@ is sometimes called the rational osmotic @C01124@.