Editing Molar volume (section)

== Definition ==
[[File:Excess Volume Mixture of Ethanol and Water-int.svg|thumb|Change in volume with increasing ethanol fraction.]]
The molar volume of a substance ''i'' is defined as its [[molar mass]] divided by its density ''ρ''<sub>''i''</sub><sup>0</sup>:
<math display="block">V_{\rm m,i} = {M_i\over\rho_i^0}</math>
For an [[ideal mixture]] containing ''N'' components, the molar volume of the mixture is the [[weighted sum]] of the molar volumes of its individual components. For a real mixture the molar volume cannot be calculated without knowing the density:
<math display="block">V_{\rm m} = \frac{\displaystyle\sum_{i=1}^{N} x_i M_i}{\rho_{\mathrm{mixture}}}</math>
There are many liquid–liquid mixtures, for instance mixing pure [[ethanol]] and pure [[water]], which may experience contraction or expansion upon mixing. This effect is represented by the quantity '''excess volume''' of the mixture, an example of [[excess property]].

=== Relation to specific volume ===
Molar volume is related to [[specific volume]] by the product with [[molar mass]]. This follows from above where the specific volume is the [[reciproc]]al of the density of a substance:
<math display="block">V_{\rm m,i} = {M_i \over \rho_i^0} = M_i v_i</math>