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Vapor 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. vapor density = mass of ''n'' molecules of gas / mass of ''n'' molecules of hydrogen Therefore: vapour density = molar mass of gas / molar mass of H2 vapour density = molar mass of gas / 2.016 vapour density = ~½ × molar mass (and thus: molar mass = ~2 × vapour density) ==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.〔(MSDS Glossary of Terms – Vapor Density ). Msdsonline.com. Retrieved on 2012-02-09.〕 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.〔(HazMat Math: Calculating Vapor Density ). Firenuggets.com. Retrieved on 2012-02-09.〕 For example, acetone has a vapour density of 2〔(MSDS: Acetone ). Hazard.com (1998-04-21). Retrieved on 2012-02-09.〕 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〔(NIOSH Pocket Guide: Acetone ). Cdc.gov. Retrieved on 2012-02-09.〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vapour density」の詳細全文を読む スポンサード リンク
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