Editing Molar volume (section)

== Crystalline solids ==
For [[Crystal|crystalline solids]], the molar volume can be measured by [[X-ray crystallography]].
The [[unit cell]] volume (''V''<sub>cell</sub>) may be calculated from the [[unit cell]] parameters, whose determination is the first step in an X-ray crystallography experiment (the calculation is performed automatically by the structure determination software). This is related to the molar volume by
<math display="block">V_{\rm m} = {{N_{\rm A}V_{\rm cell}}\over{Z}}</math>
where ''N''<sub>A</sub> is the [[Avogadro constant]] and ''Z'' is the number of formula units in the unit cell. The result is normally reported as the "crystallographic density".

=== Molar volume of silicon ===
{{See also|Avogadro project}}
Ultra-pure [[silicon]] is routinely made for the [[electronics industry]], and the measurement of the molar volume of silicon, both by X-ray crystallography and by the ratio of molar mass to mass density, has attracted much attention since the pioneering work at [[NIST]] in 1974.<ref>{{cite journal | last=Deslattes | first=R. D. |author2=Henins, A. |author3=Bowman, H. A. |author4=Schoonover, R. M. |author5=Carroll, C. L. |author6=Barnes, I. L. |author7=Machlan, L. A. |author8=Moore, L. J. |author9= Shields, W. R. | year=1974 | journal=[[Physical Review Letters|Phys. Rev. Lett.]] | volume=33 | issue=8 | pages=463–66 | doi = 10.1103/PhysRevLett.33.463
 | title = Determination of the Avogadro Constant | bibcode=1974PhRvL..33..463D}}</ref> The interest stems from that accurate measurements of the unit cell volume, [[atomic weight]] and mass density of a pure crystalline solid provide a direct determination of the Avogadro constant.<ref name="CODATA98">{{cite journal |author-first1=Peter J. |author-last1=Mohr |author-first2=Barry N. |author-last2=Taylor |title=CODATA recommended values of the fundamental physical constants: 1998 |journal=[[Journal of Physical and Chemical Reference Data]] |volume=28 |issue=6 |pages=1713–1852 |bibcode=1999JPCRD..28.1713M |doi=10.1063/1.556049 |year=1999 |url=https://www.nist.gov/pml/div684/fcdc/upload/rmp1998-2.pdf |archive-url=https://web.archive.org/web/20171001122752/https://www.nist.gov/sites/default/files/documents/pml/div684/fcdc/rmp1998-2.pdf|archive-date=2017-10-01}}</ref>

The CODATA recommended value for the molar volume of silicon is {{physconst|VmSi|ref=no}}, with a relative standard uncertainty of {{physconst|VmSi|runc=yes|after=.}}