{{Short description|Density of a vapor in relation to that of hydrogen}} '''Vapour density''' is the density of a vapour in relation to that of hydrogen. It may be defined as mass of a certain volume of a substance divided by mass of same volume of hydrogen.
:vapour density = mass of ''n'' molecules of gas / mass of ''n'' molecules of hydrogen gas . :vapour density = molar mass of gas / molar mass of H<sub>2</sub> :vapour density = molar mass of gas / 2.01568 :vapour density = {{frac|1|2}} × molar mass
(and thus: molar mass = ~2 × vapour density) For example, vapour density of mixture of NO<sub>2</sub> and N<sub>2</sub>O<sub>4</sub> is 38.3. Vapour density is a dimensionless quantity.
Vapour density = density of gas / density of hydrogen (H2)
==Alternative definition==
In many web sources, particularly in relation to safety considerations at commercial and industrial facilities in the U.S., vapour density is defined with respect to air, not hydrogen.<ref>[http://www.msdsonline.com/CustomerSupport/GlossaryOfTerms/VaporDensity.aspx MSDS Glossary of Terms – Vapour Density]. Msdsonline.com. Retrieved on 2012-02-09.</ref> Air is given a vapour density of one. For this use, air has a molecular weight of 28.97 atomic mass units, and all other gas and vapour molecular weights are divided by this number to derive their vapour density.<ref>[http://www.firenuggets.com/x_ARDSIllIIlllllIIIllIlIlIIIllllIllIl/peterson9.htm HazMat Math: Calculating Vapor Density] {{Webarchive|url=https://web.archive.org/web/20110711003737/http://www.firenuggets.com/x_ARDSIllIIlllllIIIllIlIlIIIllllIllIl/peterson9.htm |date=2011-07-11 }}. Firenuggets.com. Retrieved on 2012-02-09.</ref> For example, acetone has a vapour density of 2<ref>{{cite web |url=http://hazard.com/msds/mf/baker/baker/files/a0446.htm |title=MSDS: Acetone |date=1998-04-21 |website=hazard.com |access-date=2012-02-09 |url-status=usurped |archive-url=https://web.archive.org/web/20210923180616/http://hazard.com/msds/mf/baker/baker/files/a0446.htm |archive-date=2021-09-23}}</ref> in relation to air. That means acetone vapour is twice as heavy as air. This can be seen by dividing the molecular weight of Acetone, 58.1<ref>[https://www.cdc.gov/niosh/npg/npgd0004.html NIOSH Pocket Guide: Acetone]. Cdc.gov. Retrieved on 2012-02-09.</ref> by that of air, 28.97, which equals 2.
With this definition, the vapour density would indicate whether a gas is denser (greater than one) or less dense (less than one) than air. The density has implications for container storage and personnel safety—if a container can release a dense gas, its vapour could sink and, if flammable, collect until it is at a concentration sufficient for ignition. Even if not flammable, it could collect in the lower floor or level of a confined space and displace air, possibly presenting an asphyxiation hazard to individuals entering the lower part of that space.
==See also== *Relative density (also known as specific gravity) *Victor Meyer apparatus
==References== {{Reflist}}
{{Authority control}} Category:Density Category:Gases
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