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Deduction of relationship between molecular mass and vapour density

Relative Molecular Mass

Relative molecular mass is the ratio of the mass of one molecule of a substance to the mass th of a carbon atom, or 1 amu.

Vapour Density

Vapour density is the ratio of the mass of a volume of a gas, to the mass of an equal volume of hydrogen, measured under the same conditions of temperature and pressure.

According to Avogadro's law, equal volumes of all gases contain equal number of molecules.

Thus, let the number of molecules in one volume = n,

Cancelling 'n' which is common, we get

However, since hydrogen is diatomic.

When we compare the formulae of vapour density with relative molecular molecular mass, they can be represented as

We can therefore substitute the above equation, and arrive at the below formula

Now on cross multiplication, we have

2 x vapour density = Relative Molecular Mass of a gas

relationship between molecular mass vapour density

or,

Relative Molecular Mass = 2 x Vapour density

25 comments:

  1. UnknownApril 22, 2016 at 9:01 AM

    Thank You so much Sir.This was very helpful to me in my Mole Concept chapter.

    ReplyDelete
    Replies
    1. BoopJanuary 2, 2017 at 1:37 AM

      You are right. That helped me too in my mole concept chapter.

      Delete
    2. UnknownJune 7, 2017 at 10:09 PM

      Me too, found it quite vexing before accessing this webpage.

      Delete
    3. UnknownSeptember 4, 2017 at 11:19 PM

      Really it was helping and helped me a lot in solving problems related to gases

      Delete
  2. UnknownMay 17, 2016 at 5:35 AM

    Sir please help
    2.32g of a hydrocarbon on ignition gave 7.3g of carbon(iv)oxide and 2.98g of water. If the vapour density of the hydrocarbon is 28. Calculate
    I. The empirical formular
    2. Molecular formular
    3. Structural formular
    4 draw the structures of the possible isomers and give the IUPAC name

    ReplyDelete
    Replies
    1. UnknownJuly 7, 2016 at 8:38 PM

      Butene
      C4H4

      Delete
    2. UnknownFebruary 11, 2017 at 9:29 PM

      Butene is C4H8

      Delete
    3. aanyaApril 17, 2017 at 8:29 PM

      Butene c4h8 molecular formula
      Ch3chchch3,ch2chch2ch3,cyclobutane,methylcyclopropane

      Delete
    4. aanyaApril 17, 2017 at 8:29 PM

      Butene c4h8 molecular formula
      Ch3chchch3,ch2chch2ch3,cyclobutane,methylcyclopropane

      Delete
  3. UnknownJune 10, 2016 at 10:03 AM

    Thank you

    ReplyDelete
  4. UnknownJune 10, 2016 at 10:03 AM

    Thank u

    ReplyDelete
  5. UnknownJuly 24, 2016 at 12:33 AM

    If an equimolar mixture of NO and O2 are placed, then what is the vapour pressure of the mixture

    ReplyDelete
  6. UnknownDecember 13, 2016 at 1:37 PM

    Nice and it is too easy method of deduction of the relation of vappour density and molecular....

    ReplyDelete
  7. UnknownDecember 13, 2016 at 1:37 PM

    Nice and it is too easy method of deduction of the relation of vappour density and molecular....

    ReplyDelete
  8. anonymousFebruary 19, 2017 at 5:42 AM

    I must say, it is a highly relevant and useful article. Thank you sir!

    ReplyDelete
  9. UnknownMay 14, 2017 at 4:25 AM

    Nnah ehh theses things are confusing me please where do a learner start from

    ReplyDelete
  10. UnknownMay 14, 2017 at 4:26 AM

    Please where do a learner like starts from? Am so confused please all should help

    ReplyDelete
  11. Karthik BMay 14, 2017 at 10:31 PM

    Useful , Thanks

    ReplyDelete
  12. UnknownMay 19, 2017 at 1:28 AM

    Something strange is happening here:
    Vapour density is measured in kg/m3
    Relative molecular mass is dimensionless.

    So wouldn't it be strange that Relative molecular mass = 2* Vapour density ??

    ReplyDelete
    Replies
    1. UnknownMay 27, 2017 at 9:07 PM

      U r wrong vapour density doesnot hav unit it is ratio of mass upon mass ...so what u were thinking....plz be carefull

      Delete
    2. SonuMarch 29, 2018 at 12:40 AM

      Vapour density too means relative density

      Delete
  13. UnknownMay 28, 2017 at 12:28 AM

    40 ml of mixture of Co and H2 yielded 15 ml of Co2 after explosion with excess O2 The ℅of H2 in the mixture

    ReplyDelete
  14. Jeet sarkarJune 21, 2017 at 7:29 PM

    According to Avogadro's hypothesis at same temp.and pressure equal volume of any gas contains equal number of molecule.
    (D)=n-molecules of gas/n-molecules of hydrogen
    =Weight of 1-molexule of gas/weight of 1-molecule of hydrogen
    =Molecular weight of gas (M)/molecular weight of hydrogen.

    ReplyDelete
  15. AnkitaApril 6, 2018 at 10:49 PM

    A mixture of methane and oxygen has vapour density 10. The % (by mole) of methane is ?? Pls help with this question sir

    ReplyDelete