Editing Metric system (section)

===Mole and mass relationship===
A mole of a substance has a mass that is its [[molecular mass]] expressed in units of grams. The mass of a mole of carbon is 12.0&nbsp;g, and the mass of a mole of table salt is 58.4&nbsp;g.

Since all gases have the same volume per mole at a given temperature and pressure far from their points of liquefaction and solidification (see [[Perfect gas]]), and air is about {{sfrac|1|5}} oxygen (molecular mass 32) and {{sfrac|4|5}} nitrogen (molecular mass 28), the density of any near-perfect gas relative to air can be obtained to a good approximation by dividing its molecular mass by 29 (because {{nowrap|{{sfrac|4|5}} × 28 + {{sfrac|1|5}} × 32 {{=}} 28.8 ≈ 29}}). For example, [[carbon monoxide]] (molecular mass 28) has almost the same density as air.